remove Plugin-Module manifest entry

[jtnord]

THe manifest entry was not correctly caclulated, being different depending on where you ran maven from and if you also had any local aggrgator project or not.

The original intention and need of this was to use the path from the git repostory for the PCT, but the code changed from using a git path along the way.

As a result the code produced non deterministic results and the PCT which needed this information has been updated to not need this in jenkinsci/plugin-compat-tester#557

because the information is non deterministic and there are no consumers of this, it is simpler to remove the code.

fixes #497

Testing done

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