NIAD-3463: Remove PI Test from gradle /CI/CD.

[MartinWheelerMT]

What

• Remove all references, licences and workflows relating to mutation testing as repositories must be made private, but our license is for open source repositories only.

Why

repositories must be made private, but our PITest license is for open source repositories only.

Type of change

Please delete options that are not relevant.

• Internal change (non-breaking change with no effect on the functionality affecting end users)

Checklist:

• I have performed a self-review of my code
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have updated the Changelog with details of my change in the UNRELEASED section if this change will affect end users
• A corresponding change has been made to the Mapping Documentation repository

[github-actions]

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