RFR: 8324769: Serial: Remove unused TenuredGeneration::unsafe_max_alloc_nogc

[Albert Mingkun Yang]

Trivial removing dead code and moving a method from supreclass to subclass.

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Commit messages:
 - s1-unsafe

Changes: https://git.openjdk.org/jdk/pull/17591/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17591&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8324769
  Stats: 19 lines in 4 files changed: 7 ins; 12 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/17591.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17591/head:pull/17591

PR: https://git.openjdk.org/jdk/pull/17591