Integrated: 8364504: [BACKOUT] JDK-8364176 Serial: Group all class unloading logic at the end of marking phase

[Albert Mingkun Yang]

A clean revert of  "JDK-8364176 Serial: Group all class unloading logic at the end of marking phase".

Test: Used `jdk/jfr/event/gc/stacktrace/TestMetaspaceSerialGCAllocationPendingStackTrace.java` to verify that it fails before the revert and passes after the revert.

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

Commit messages:
 - backout

Changes: https://git.openjdk.org/jdk/pull/26587/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26587&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8364504
  Stats: 22 lines in 2 files changed: 10 ins; 9 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/26587.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26587/head:pull/26587

PR: https://git.openjdk.org/jdk/pull/26587