RFR: 8217474: Remove WhiteBox.getConcurrentGCPhases()

[Kim Barrett]

Please review this change to remove a not really useful test and the
assocated infrastructure.  The test really doesn't carry its weight.

This is a step toward improving the WhiteBox control of concurrent
collection cycles, to make it more amenable for use by ZGC too.

[I'll update copyrights before pushing.]

CR:
https://bugs.openjdk.java.net/browse/JDK-8217474

Webrev:
http://cr.openjdk.java.net/~kbarrett/8217474/open.00/

Testing:
mach5 tier1, which includes running the tests in
jtreg/gc/concurrent_phase_control.