RFR: 8335925: Serial: Move allocation API from Generation to subclasses

[Albert Mingkun Yang]

Trivial moving methods from parent class to subclasses. The unused second arg is also removed along the way. The API names are descriptive enough so that the accompanying comments are dropped as well.

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Commit messages:
 - s1-gen-alloc

Changes: https://git.openjdk.org/jdk/pull/20084/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20084&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8335925
  Stats: 43 lines in 7 files changed: 0 ins; 21 del; 22 mod
  Patch: https://git.openjdk.org/jdk/pull/20084.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20084/head:pull/20084

PR: https://git.openjdk.org/jdk/pull/20084