RFR: 8301465: Remove unnecessary nmethod arming in Full GC of Serial/Parallel

[Albert Mingkun Yang]

Simple removing unnecessary code and adding API documentation.

Test: hotspot_gc

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Commit messages:
 - arm-nmethod

Changes: https://git.openjdk.org/jdk/pull/12311/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12311&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8301465
  Stats: 8 lines in 3 files changed: 6 ins; 2 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/12311.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12311/head:pull/12311

PR: https://git.openjdk.org/jdk/pull/12311