RFR: 8264742: member variable _monitor of MonitorLocker is redundant

[Xin Liu]

MonitorLocker is a subclass of MutexLocker. The member variable _monitor
has the same value of _mutex in its base class.

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Commit messages:
 - member variable _monitor of MonitorLocker is redundant

Changes: https://git.openjdk.java.net/jdk/pull/3350/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3350&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264742
  Stats: 15 lines in 1 file changed: 5 ins; 1 del; 9 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3350.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3350/head:pull/3350

PR: https://git.openjdk.java.net/jdk/pull/3350