Do not patch out display-manager.service alias

This is needed so that presets can enable the manager, and also as a pre-greeter synchronization point (eg: for firstboot services that provision users). The custom debconf-based mechanism does not conflict with this, as it does the same thing: create and remove symlinks.

Other managers already use it and leave it in place:

https://sources.debian.org/src/lightdm/1.32.0-6/debian/lightdm.service https://sources.debian.org/src/sddm/0.21.0%2Bgit20250502.4fe234b-2/debian/sddm.service https://sources.debian.org/src/wdm/1.28-27/debian/wdm.service

Closes: #1025349