RFR: 8371852: Shenandoah: Remove unused ShenandoahFreeSet::_allocated_sinc…

[Nityanand Rai]

Removed the unused _allocated_since_gc_start[UIntNumPartitions] field from ShenandoahRegionPartitions class. This field became obsolete after JDK-8365880 unified memory usage accounting in ShenandoahFreeSet but was not cleaned up.

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Commit messages:
 - 8371852: Shenandoah: Remove unused ShenandoahFreeSet::_allocated_since_gc_start field

Changes: https://git.openjdk.org/jdk/pull/28332/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28332&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8371852
  Stats: 3 lines in 1 file changed: 0 ins; 3 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/28332.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28332/head:pull/28332

PR: https://git.openjdk.org/jdk/pull/28332