RFR: 8335618: Serial: Remove unused definitions in SerialHeap

[Albert Mingkun Yang]

Trivial removing dead code.

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Commit messages:
 - trivial

Changes: https://git.openjdk.org/jdk/pull/20005/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20005&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8335618
  Stats: 8 lines in 2 files changed: 0 ins; 8 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/20005.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20005/head:pull/20005

PR: https://git.openjdk.org/jdk/pull/20005