RFR: 8368752: Serial: Remove unused arg of DefNewGeneration::gc_epilogue

[Albert Mingkun Yang]

Trivial removing dead code.

Test: tier1

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

Commit messages:
 - sgc-trivial

Changes: https://git.openjdk.org/jdk/pull/27516/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27516&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8368752
  Stats: 3 lines in 3 files changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/27516.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27516/head:pull/27516

PR: https://git.openjdk.org/jdk/pull/27516