RFR: 8205028: ZGC: Remove incorrect comment in ZHeap::object_iterate()

[Per Liden]

The comment in ZHeap::object_iterate() saying "Should only be called in 
a safepoint after mark end" is incorrect and should be removed. It's 
still true that this should only be called in a safepoint (which the 
assert below checks), but the part about "after mark end" is not true.

Bug: https://bugs.openjdk.java.net/browse/JDK-8205028
Webrev: http://cr.openjdk.java.net/~pliden/8205028/webrev.0

/Per