RFR: 8364110: Remove unused methods in GCCause

[Albert Mingkun Yang]

Trivial removing dead code.

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Commit messages:
 - gc-cause-trivial-remove

Changes: https://git.openjdk.org/jdk/pull/26476/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26476&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8364110
  Stats: 20 lines in 1 file changed: 0 ins; 20 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/26476.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26476/head:pull/26476

PR: https://git.openjdk.org/jdk/pull/26476