It's polkit.service, not polkitd.service.

[tom@archlinux ~]$ cat /usr/lib/systemd/system/polkit.service 
[Unit]
Description=Authorization Manager
Documentation=man:polkit(8)

[Service]
Type=dbus
BusName=org.freedesktop.PolicyKit1
ExecStart=/usr/lib/polkit-1/polkitd --no-debug
[tom@archlinux ~]$ 

Tip: You could have found out with dpkg -L policykit-1 | grep service or even the lists of files of a package on https://packages.ubuntu.com/.

polkit (formerly Policy Kit) provides two important services.

From the project documentation:

an authorization API intended to be used by privileged programs (“MECHANISMS”) offering service to unprivileged programs (“SUBJECTS”) often through some form of inter-process communication mechanism.

...and...

allows users to obtain temporary authorization through authenticating either an administrative user or the owner of the session the client belongs to.

The first provides a safe way for user-level applications to ask for root-level actions. You might use this regularly without noticing in Desktop systems -- like suspending, or mounting newly-inserted hardware. Polkit is part of that mysterious glue that makes the desktop work.

The second is (most commonly) the GUI "admin privileges are required for this actions" dialog.

polkit is a necessary element in all Ubuntu Desktop (GUI) systems. Removing polkit will destroy your Ubuntu Desktop (GUI) system. It's generally not needed if you run headless (without GUI / Ubuntu Server), though folks do sometimes add software that needs polkit.