RFR: 8325134: Serial: Remove Generation::used_region

[Albert Mingkun Yang]

Trivial removing dead code. The change of moving the definition to `tenuredGeneration.hpp` is needed to work around linking failure.

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Commit messages:
 - trivial

Changes: https://git.openjdk.org/jdk/pull/17673/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17673&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8325134
  Stats: 10 lines in 3 files changed: 1 ins; 9 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/17673.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17673/head:pull/17673

PR: https://git.openjdk.org/jdk/pull/17673