RFR: 8260415: Remove unused class ReferenceProcessorMTProcMutator

[Stefan Johansson]

On Mon, 15 Feb 2021 15:51:30 GMT, Leo Korinth <lkorinth at openjdk.org> wrote:

> ReferenceProcessorMTProcMutator is not used. ReferenceProcessorMTDiscoveryMutator seems to do the same and is still being used.

Looks good and trivial.

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Marked as reviewed by sjohanss (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/2578