core: don't manually destroy timer when we can't spawn a child

core: don't manually destroy timer when we can't spawn a child

Let's stop manually destroying the timers when we fail to spawn a child.
We don't do this in any of the similar codepaths in any of the unit
types, only in two specific ones in socket/swap. Destroying the timer is
unnecessary, since this is done anyway in the _set_state() call of each
unit type if not appropriate, and every failure path here runs through
that anyway.

This brings all these similar codepaths into sync.