RFR: 8343730: JMX cleanups

[Kevin Walls]

A few small cleanup changes in javax/management.
No logic changes, comments and text only.

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Commit messages:
 - 8343730: JMX cleanups

Changes: https://git.openjdk.org/jdk/pull/21938/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21938&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8343730
  Stats: 12 lines in 4 files changed: 0 ins; 5 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/21938.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21938/head:pull/21938

PR: https://git.openjdk.org/jdk/pull/21938