RFR: 8309286: G1: Remove unused G1HeapRegionAttr::is_valid_gen

[Albert Mingkun Yang]

Trivial removing dead code.

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Commit messages:
 - trivial

Changes: https://git.openjdk.org/jdk/pull/14260/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14260&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8309286
  Stats: 1 line in 1 file changed: 0 ins; 1 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/14260.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14260/head:pull/14260

PR: https://git.openjdk.org/jdk/pull/14260