RFR: 8310311: Serial: move Generation::contribute_scratch to DefNewGeneration

[Albert Mingkun Yang]

Simple cleanup to move a method from base-class to  sub-class.

Test: hotspot_gc

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Commit messages:
 - s1-contribute-scratch

Changes: https://git.openjdk.org/jdk/pull/14540/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14540&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8310311
  Stats: 24 lines in 4 files changed: 4 ins; 13 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/14540.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14540/head:pull/14540

PR: https://git.openjdk.org/jdk/pull/14540