Revert "Handle multiple display managers which don't ship a systemd unit, part 2"

Revert "Handle multiple display managers which don't ship a systemd unit, part 2"

This reverts commits 9868be7730f and 2e3f2d06a4a3.

This is still under dispute, a bit risky, and might get a different
implementation. Also, nodm really needs to be fixed properly, working around it
is both too risky and also too hard to get right.