units: remove udev control socket when systemd stops the socket unit (#4039)

Mere presence of the socket in the filesystem makes
udev_queue_get_udev_is_active() return that udev is running. Note that,
udev on exit doesn't unlink control socket nor does systemd. Thus socket
stays around even when both daemon and socket are stopped. This causes
problems for cryptsetup because when it detects running udev it launches
synchronous operations that *really* require udev. This in turn may
cause blocking and subsequent timeout in systemd-cryptsetup on reboot
while machine is in a state that udev and its control socket units are
stopped, e.g. emergency mode.

Fixes #2477