nixos/lightdm: do not conflict with plymouth (#71061)

Having `display-manager` conflict with `plymouth-quit` causes this lock up:

 - `plymouth-quit-wait` starts up, waiting for plymouth-quit to run
 - `lightdm` starts up
 - `plymouth-quit` can't start, it conflicts with lightdm
 - `plymouth-quit-wait` keeps waiting on plymouth-quit to kill plymouthd

The idea is having LightDM control when plymouth quits, but communication with
plymouth was broken: https://github.com/NixOS/nixpkgs/pull/71064

Unfortunately having the conflict breaks switching to configurations with
plymouth enabled. So we still need to remove the conflict.

fixes #71034