RFR: 8234437: Remove CollectedHeap::safe_object_iterate()

[Per Liden]

With CMS now removed, there is no longer any GC where 
CollectedHeap::object_iterate() and CollectedHeap::safe_object_iterate() 
do different things. Hence, we can remove 
CollectedHeap::safe_object_iterate() and let all uses of it shift over 
to use CollectedHeap::object_iterate().

Bug: https://bugs.openjdk.java.net/browse/JDK-8234437
Webrev: http://cr.openjdk.java.net/~pliden/8234437/webrev.0

/Per