RFR [S][5/7]: 8197573: Remove concurrent cleanup and secondary free list handling

[Thomas Schatzl]

Hi all,

  can I have reviews for this change that removes the concurrent
cleanup phase and the secondary free list handling entirely. It moves
leftover work into the Cleanup pause.

In the previous change the remembered set scrubbing, which took in
cases where it mattered like 95%+ of time, has been removed because it
is not required any more.

The reasons for removal are:
- due to JDK-8180415 *freeing* remembered sets (as opposed to
calculating occupancy) is really fast, so this phase has never been
very long for a long time.
- it adds a lock in the LAB allocation path

It also allows us to remove the "mixed_gc_pending" flag in the
CollectorState, as there is no further time to wait until G1 can
schedule the "last young gc".

CR:
https://bugs.openjdk.java.net/browse/JDK-8197573
Webrev:
http://cr.openjdk.java.net/~tschatzl/8197573/webrev/index.html
Testing:
hs-tier 1-3, ....

(I consider this an "S" changeset because it is mostly removal of
code).

Thanks,
  Thomas