RFR: 8320877: Shenandoah: Remove ShenandoahUnloadClassesFrequency support

Tue Nov 28 17:52:21 UTC 2023

Flag cleanup. `ShenandoahUnloadClassesFrequency` was added long time ago when Shenandoah did not support concurrent class unloading. We kept it in to simplify backports. At this point, most releases support concurrent class unloading, and there is no reason to keep the flag in. The default is "1" since [JDK-8228720](https://bugs.openjdk.org/browse/JDK-8228720).

Additional testing:
 - [x] MacOS AArch64 server fastdebug, `hotspot_gc_shenandoah`

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

Commit messages:
 - Revert incorrect cleanup
 - More cleanups
 - Fix

Changes: https://git.openjdk.org/jdk/pull/16847/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16847&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8320877
  Stats: 37 lines in 5 files changed: 0 ins; 34 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/16847.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16847/head:pull/16847

PR: https://git.openjdk.org/jdk/pull/16847