unit: rework trigger dependency logic

Instead of having explicit type-specific callbacks that inform the
triggering unit when a triggered unit changes state, make this generic
so that state changes are forwarded betwee any triggered and triggering
unit.

Also, get rid of UnitRef references from automount, timer, path units,
to the units they trigger and rely exclsuively on UNIT_TRIGGER type
dendencies.

src/core/timer.h

@@ -74,7 +74,6 @@ struct Timer {
         usec_t next_elapse_realtime;
 
         TimerState state, deserialized_state;
-        UnitRef unit;
 
         Watch monotonic_watch;
         Watch realtime_watch;
@@ -82,8 +81,6 @@ struct Timer {
         TimerResult result;
 };
 
-void timer_unit_notify(Unit *u, UnitActiveState new_state);
-
 void timer_free_values(Timer *t);
 
 extern const UnitVTable timer_vtable;