RFR: 8325134: Serial: Remove Generation::used_region [v2]

[Albert Mingkun Yang]

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

Albert Mingkun Yang has updated the pull request incrementally with one additional commit since the last revision:

  review

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Changes:
  - all: https://git.openjdk.org/jdk/pull/17673/files
  - new: https://git.openjdk.org/jdk/pull/17673/files/7c281dcc..12b31e68

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=17673&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=17673&range=00-01

  Stats: 1 line in 1 file changed: 1 ins; 0 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