RFR: 8325871: Move StringTable and SymbolTable rehashing calls

[Coleen Phillimore]

Move the rehashing calls as a test in the concurrent work - ie. trigger concurrent work and check for the need to do rehashing. 

Tested with tier1-7.

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Commit messages:
 - Remove now unused Cleanup VM_Operation.
 - 8325871: Move StringTable and SymbolTable rehashing calls
 - String Table

Changes: https://git.openjdk.org/jdk/pull/18240/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18240&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8325871
  Stats: 228 lines in 10 files changed: 78 ins; 117 del; 33 mod
  Patch: https://git.openjdk.org/jdk/pull/18240.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18240/head:pull/18240

PR: https://git.openjdk.org/jdk/pull/18240