Revert "units: skip modprobe@.service if the unit appears to be already loaded"

Revert "units: skip modprobe@.service if the unit appears to be already loaded"

This reverts commit 9cbf1e58f9629af5c6b56777ee73dc6320306d6d.

The presence of /sys/module/%I directory can't be used to assert that the load
of a given module is complete and therefore the call to modprobe(8) can be
skipped. Indeed this directory is created before the init() function of the
module is called.

Users of modprobe@.service needs to be sure that once this service returns the
module is fully operational.

(cherry picked from commit b3e32582f67c68896b6b05893b138a975fa26454)