RFR: 8351700: Remove code conditional on BarrierSetNMethod being null [v3]

[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.

Doug Simon has updated the pull request incrementally with one additional commit since the last revision:

  removed unused code

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Changes:
  - all: https://git.openjdk.org/jdk/pull/23996/files
  - new: https://git.openjdk.org/jdk/pull/23996/files/b3d4721d..95da3c2f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=23996&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=23996&range=01-02

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