RFR: 8315446: G1: Remove unused G1AllocRegion::attempt_allocation

[Albert Mingkun Yang]

Trivial removing dead code.

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Commit messages:
 - trivial

Changes: https://git.openjdk.org/jdk/pull/15509/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15509&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8315446
  Stats: 9 lines in 2 files changed: 0 ins; 9 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/15509.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15509/head:pull/15509

PR: https://git.openjdk.org/jdk/pull/15509