RFR: 8253815: Remove unused HeapRegionManager::_num_committed from SA

[Stefan Johansson]

While doing some refactoring I wanted to move HeapRegionManager::_num_committed and realized that I needed to update
the SA. After some looking around it turns out that it is unused and I can remove the numCommittedField from the
HeapRegionManager class in the SA.

Local build and test looks good and running tier1 and some svc-testing to ensure I haven't missed anything.

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Commit messages:
 - 8253815: Remove unused HeapRegionManager::_num_committed from SA

Changes: https://git.openjdk.java.net/jdk/pull/408/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=408&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8253815
  Stats: 8 lines in 2 files changed: 0 ins; 8 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/408.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/408/head:pull/408

PR: https://git.openjdk.java.net/jdk/pull/408