RFR: 8291626: Remove Mutex::contains as it is unused

[Johan Sjölén]

Hi,

Please review this PR removing some dead code.

Passed tier1-3 against commit with 2a1d9cf as parent, running tier1-2 on top of HEAD.

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Commit messages:
 - Remove Mutex::contains

Changes: https://git.openjdk.org/jdk/pull/9701/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9701&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8291626
  Stats: 10 lines in 2 files changed: 0 ins; 10 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/9701.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9701/head:pull/9701

PR: https://git.openjdk.org/jdk/pull/9701