RFR: 8242597: Remove GenericTaskQueue<>::push_slow

[Kim Barrett]

Please review this change which simplifies GenericTaskQueue::push() by
removing some provably unnecessary code.  The case that push_slow was
handling, where dirty_size == N-1, cannot occur.  Also added or
updated some related comments.

CR:
https://bugs.openjdk.java.net/browse/JDK-8242597

Webrev:
https://cr.openjdk.java.net/~kbarrett/8242597/open.00/

Testing:
mach5 tier1-7