RFR: 8264050: Remove unused field VM_HeapWalkOperation::_collecting_heap_roots

[Aleksey Shipilev]

SonarCloud reports field `_collecting_heap_roots` is not initialized after constructor ends. In fact, that field is not used anywhere. It was like that since the initial load. We can trivially remove it.

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Commit messages:
 - 8264050: Remove unused field VM_HeapWalkOperation::_collecting_heap_roots

Changes: https://git.openjdk.java.net/jdk/pull/3153/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3153&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264050
  Stats: 1 line in 1 file changed: 0 ins; 1 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3153.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3153/head:pull/3153

PR: https://git.openjdk.java.net/jdk/pull/3153