RFR: 8352773: JVMTI should disable events during java upcalls

[Chris Plummer]

Calling ThreadGroupReference.groups() from an event handler can cause a deadlock. Details in first comment. Tested with :jdk_lang on all supported platforms and tier1, tier2, tier3, and tier5 svc testing.

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

Commit messages:
 - get rid of unused breakpoint event code
 - fix more jcheck errors
 - fix jcheck errors
 - Avoid class loading in ThreadGroup.subgroupsAsArray()

Changes: https://git.openjdk.org/jdk/pull/24236/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24236&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8352773
  Stats: 126 lines in 2 files changed: 124 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/24236.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24236/head:pull/24236

PR: https://git.openjdk.org/jdk/pull/24236