RFR: 8337642: Serial: Remove redundant counter update in DefNewGeneration::gc_epilogue

[Albert Mingkun Yang]

Trivial removing an empty method call. (Only subclasses have non-empty method body, which is not used by Serial.)

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Commit messages:
 - s1-trivial

Changes: https://git.openjdk.org/jdk/pull/20416/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20416&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8337642
  Stats: 1 line in 1 file changed: 0 ins; 1 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/20416.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20416/head:pull/20416

PR: https://git.openjdk.org/jdk/pull/20416