Use this:

<ul>
    <li><input type="radio" name="list" value="volvo">Volvo <button>delete</button></li>
    <li><input type="radio" name="list" value="saab">Saab <button>delete</button></li>
    <li><input type="radio" name="list" value="mercedes">Mercedes <button>delete</button></li>
    <li><input type="radio" name="list" value="mercedes">Mercedes <button>delete</button></li>
</ul>

Then add event listeners to each button that removes the parent <li> from the list (Demo at jsfiddle.net).

Your delete selector is wrong, try .card .delete:

$(document).ready(function() {
  $("button.plus").on("click", function() {
    var newCard = $('#cardPrototype').clone(true);
    $(newCard).css('display', 'block').removeAttr('id');
    $('#newCardHolder').append(newCard);
  });

  $('body').on('click', '.card', function() {
    $(this).find('form').show();
    $(this).find('span').remove();

  });

  /*delete button*/
  $('body').on('click', '.card .delete', function() {
    $(this).closest('.card').remove();
  });
});

.input-feld {
  font-family: TheSans Swisscom;
  margin: 3px;
  width: 260px;
}
.card {
  width: 300px;
  margin-right: 5px;
  margin-left: 5px;
  margin-bottom: 10px;
  float: left;
  padding: 10px;
  background-color: #BBBBBB;
  border: 1px solid #ccc;
  border-radius: 10px;
  position: relative;
}
.delete {
  font-family: 'TheSans Swisscom';
  right: 12px;
  top: 0;
  position: absolute;
}
.speichern {
  font-family: 'TheSans Swisscom';
  background-color: greenyellow;
}
.abbrechen {
  font-family: "TheSans Swisscom";
  background-color: red;
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<button class="plus">
  <div class="item">
    <p>+</p>

  </div>
</button>

<div id="newCardHolder">

</div>

<div id="cardPrototype" class="card" style="display:none;">
  <p class="delete">x</p>
  <span>Title</span>
  <form name="theform" style="display:none;">
    <input class="input-feld" type="text">
    <br>
    <input class="input-feld " type="text">
    <br>
    <input class="speichern" type="button" onClick="new Person()" value="Speichern">
    <input class="abbrechen" type="button" onClick="new Person()" value="Abbrechen">
  </form>
</div>

Why not isolate the forms and pass the ID to the action page?

<table>
    <tr>
        <td>Item 1</td>
        <td><form action='action.php?id=1' method='post'><input type='submit' value='X'></form></td>
    </tr>
    <tr>
        <td>Item 2</td>
        <td><form action='action.php?id=2' method='post'><input type='submit' value='X'></form></td>
    </tr>
</table>

First of all u can't repeat names so u should change delt function's name

As a recomendation try to name functions according it's functionality

function deleteItem(e){
  console.log("Button element", e)
  e.parentElement.remove()
}

<ul>
  <li>Item 1 <button onclick="deleteItem(this)">Delete</button></li>
  <li>Item 2 <button onclick="deleteItem(this)">Delete</button></li>
  <li>Item 3 <button onclick="deleteItem(this)">Delete</button></li>
  <li>Item 4 <button onclick="deleteItem(this)">Delete</button></li>
  <li>Item 5 <button onclick="deleteItem(this)">Delete</button></li>
</ul>

By writing deleteItem(this) you pass element object to function

If you want your button to pass event to function then just change this for event

Btw, it's not a good practice to have your HTML && JS code in the same place, so you should do it this way

window.addEventListener("DOMContentLoaded", function(event){
  // Once DOM is loaded add onclick function
  // Select all buttons with class "btn-delete"
  document.querySelectorAll('button.btn-delete').forEach(el => {
    // Walk each selected button and set onclick function
    // remember to declare event function within a wrapper if you're gonna pass a var
    el.onclick = () => deleteItem(el)
    /* "() => Code" is called arrow function
    * it's the same as a regular anonymous function
      function(){Code}
    */
    // if you want to just execute the function
    // el.onclick = deleteItem
  })
});
// Declare your function outer the DOMContentLoaded event listener
function deleteItem(e){
  // e is the element object, so you do not need to get .target
  e.parentElement.remove()
}

<ul>
  <li>Item 1 <button class="btn-delete">Delete</button></li>
  <li>Item 2 <button class="btn-delete">Delete</button></li>
  <li>Item 3 <button class="btn-delete">Delete</button></li>
  <li>Item 4 <button class="btn-delete">Delete</button></li>
  <li>Item 5 <button class="btn-delete">Delete</button></li>
</ul>

They both works the same, but it'd be better for you to get used to that organization so when your code grows up it would not be spaghetti code

If you want to know more about that check this link