RFR: 8280817: Clean up and unify empty VM operations

[Stefan Karlsson]

There are a number of VM operations that do nothing, except triggering a safepoint. I'd like to clean up and unify them a bit to:
1) Use one parent class which implements the empty doit function and turns off thread oop processing.
2) Remove unused VM_Operations
3) Don't reuse the VM_None type - there's bug/inconsistency here in that some subsystems report the name as None, while others report the name passed to the constructor
4) Remove unused enum values

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Commit messages:
 - 8280817: Clean up and unify empty VM operations

Changes: https://git.openjdk.java.net/jdk/pull/7261/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7261&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8280817
  Stats: 59 lines in 4 files changed: 10 ins; 24 del; 25 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7261.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7261/head:pull/7261

PR: https://git.openjdk.java.net/jdk/pull/7261