RFR: 8291056: Remove unused Generation::par_promote*()

[Zhengyu Gu]

Please review this trivial cleanup to remove unused Generation::par_promote() and Generation::par_promote_alloc_done() methods.

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Commit messages:
 - Removed unused par_promote_xxx methods

Changes: https://git.openjdk.org/jdk/pull/9648/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9648&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8291056
  Stats: 21 lines in 2 files changed: 0 ins; 21 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/9648.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9648/head:pull/9648

PR: https://git.openjdk.org/jdk/pull/9648