RFR: 8373054: Shenandoah: Remove unnecessary BarrierSetNMethod::arm in shenandoahCodeRoots

[Ben Taylor]

The call to arm is redundant, and can be replaced with an assert to ensure the precondition remains true.

The same set of tier1 tests pass before and after this change with a fastdebug and Shenandoah GC.

-------------

Commit messages:
 - 8373054: Shenandoah: Remove unnecessary BarrierSetNMethod::arm in shenandoahCodeRoots

Changes: https://git.openjdk.org/jdk/pull/28648/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28648&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8373054
  Stats: 3 lines in 1 file changed: 0 ins; 1 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/28648.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28648/head:pull/28648

PR: https://git.openjdk.org/jdk/pull/28648