Before the idle notifications to V8 were sent without a special
Isolate entered, which results in V8 using it's default isolate
(which we don't use at all). Hence those were pretty useless
anyways (if they were called, which was unlikely).
Besides V8 seems to not trigger the weak object callbacks if
the isolate is destroyed, hence the PHP objects we add-ref'ed
before will leak.
Therefore this removes the previous idle notification logic
and forces garbage collection from the V8Js object destructor.
