RFR: 8300012: Remove unused JDI VirtualMachineImpl.removeObjectMirror(ObjectReferenceImpl object) method

[Chris Plummer]

`VirtualMachineImpl.removeObjectMirror(ObjectReferenceImpl object)` is not used. Furthermore it confuses the reader that runs across `removeObjectMirror()` calls, because what is actually being called is `removeObjectMirror(SoftObjectReference ref)`.

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Commit messages:
 - Remove extra removeObjectMirror() method.

Changes: https://git.openjdk.org/jdk/pull/11957/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11957&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8300012
  Stats: 18 lines in 1 file changed: 0 ins; 17 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/11957.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11957/head:pull/11957

PR: https://git.openjdk.org/jdk/pull/11957