RFR(T): 8230372: Remove G1GCPhaseTimes::MergeLBProcessedBuffers

[Kim Barrett]

Please review this trivial cleanup.  We're removing a phase time work
item that is no longer used for anything except logging.  It is no
longer interesting even for that, having been superseded by other,
more useful information that is similarly logged.

CR:
https://bugs.openjdk.java.net/browse/JDK-8230372

Webrev:
http://cr.openjdk.java.net/~kbarrett/8230372/open.00/

Testing:
Local (linux-x64) hotspot:tier1.  That includes gc/g1/TestGCLogMessages.java,
which was checking for the associated