RFR(XS): 8166228: Remove unused HeapRegion::object_iterate_mem_careful()

[Kim Barrett]

Please review this removal of an unused function.

We're doing this removal now, rather than trying to figure out what to
do about its call to klass_or_null() as part of JDK-8160369 [1].

CR:
https://bugs.openjdk.java.net/browse/JDK-8166228

Webrev:
http://cr.openjdk.java.net/~kbarrett/8166228/webrev.00/

Testing:
Successful build with function removed.

[1] https://bugs.openjdk.java.net/browse/JDK-8160369
Memory fences needed around setting and reading object lengths