RFR: 8329775: Serial: Remove unused declarations in serialFullGC.hpp

[Albert Mingkun Yang]

Trivial removing dead code.

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

Commit messages:
 - trivial

Changes: https://git.openjdk.org/jdk/pull/18655/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18655&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8329775
  Stats: 13 lines in 1 file changed: 0 ins; 13 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/18655.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18655/head:pull/18655

PR: https://git.openjdk.org/jdk/pull/18655