RFR: 8297295: Remove ThreadGroup.allowThreadSuspension

[Alan Bateman]

Another small step in the multi-release/multi-year effort to remove cruft from Thread/ThreadGroup.

java.lang.ThreadGroup.allowThreadSuspension(boolean) dates from JDK 1.1 and the Classic VM. The method controlled whether threads were suspended when the GC failed. It appears to have interacted with a callback mechanism that could potentially free memory, allowing the GC to retry. The method was never specified .

The method was deprecated and changed to do nothing in JDK 1.2. It was deprecated for removal in Java 14. 

A corpus analysis of 30M classes in 130k artifacts found 0 usages of this method.

It is time to finally remove this method. The compatibility impact should be negligible. Joe, Stuart and I discussed briefly and think early in JDK 21 is a good time to do this.

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Commit messages:
 - Remove allowThreadSuspension

Changes: https://git.openjdk.org/jdk/pull/11373/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11373&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8297295
  Stats: 24 lines in 2 files changed: 0 ins; 24 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/11373.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11373/head:pull/11373

PR: https://git.openjdk.org/jdk/pull/11373