When you compare a return value to true you shouldn't use return true, just true:

function RemoveProduct() {
  if (confirm("Poista?") == true) {
    return true;
  } else {
    return false;
  }
}

You don't even need to do the comparison, as the result from confirm is a boolean value:

function RemoveProduct() {
  if (confirm("Poista?")) {
    return true;
  } else {
    return false;
  }
}

And you don't even need the if statement, you can just return the result from confirm:

function RemoveProduct() {
  return confirm("Poista?");
}

Remember to use return when you use the function in an event. Example:

<input type="submit" onclick="return RemoveProduct();" />