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

This reverts commit 9cbf1e58f9.

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.

units/modprobe@.service

@@ -13,7 +13,6 @@ DefaultDependencies=no
 Before=sysinit.target
 Documentation=man:modprobe(8)
 ConditionCapability=CAP_SYS_MODULE
-ConditionPathExists=!/sys/module/%I
 
 [Service]
 Type=oneshot