RFR: 8351700: Remove code conditional on BarrierSetNMethod being null

[Doug Simon]

All GCs started needing nmethod entry barriers as of loom so there's no longer any need to test for null nmethod entry barriers.

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Commit messages:
 - nmethod entry barriers are no longer optional

Changes: https://git.openjdk.org/jdk/pull/23996/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23996&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8351700
  Stats: 171 lines in 27 files changed: 5 ins; 103 del; 63 mod
  Patch: https://git.openjdk.org/jdk/pull/23996.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23996/head:pull/23996

PR: https://git.openjdk.org/jdk/pull/23996