Drop Add-targets-for-compatibility-with-Debian-insserv-sy.patch

We don't need $x-display-manager any more as most/all DMs ship native services,
and $mail-transport-agent is not widely used (not even by our default MTA
exim4).

See https://docs.google.com/spreadsheets/d/1yBiz8o5reUgy0VACqLYMfJkkTNzymazT2Sk7atIa128

Adjust Add-support-for-rcS.d-init-scripts-to-the-sysv-gener.patch accordingly.