RFR: 8316940: Serial: Remove unused declarations in genCollectedHeap

[Albert Mingkun Yang]

Trivial removing dead code.

-------------

Commit messages:
 - trivial

Changes: https://git.openjdk.org/jdk/pull/15913/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15913&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8316940
  Stats: 5 lines in 2 files changed: 0 ins; 5 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/15913.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15913/head:pull/15913

PR: https://git.openjdk.org/jdk/pull/15913