RFR: 8282878: Removed _JavaThread from PhaseTraceTime

[Alex Blewitt]

The `PhaseTraceTime` class has a private `JavaThread _thread` field which doesn't appear to be referenced anywhere, and builds fine without it. It's not clear if this can be removed but it builds cleanly when not present.

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Commit messages:
 - 8282878: Removed _JavaThread from PhaseTraceTime

Changes: https://git.openjdk.java.net/jdk/pull/7756/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7756&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8282878
  Stats: 1 line in 1 file changed: 0 ins; 1 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7756.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7756/head:pull/7756

PR: https://git.openjdk.java.net/jdk/pull/7756