There's a closed pull request on the void-packages repository

{gmpc,libmpd}: remove package
https://github.com/void-linux/void-packages/pull/38866

Description:
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#### Testing the changes
- I tested the changes in this PR: **briefly**

gmpc is unmaintained and hasn't seen a release since 2011 and the homepage is now a parked domain.

This is necessary to remove libunique1/libunique which have been deprecated for a long time and are erroneously included by a few packages (will be fixed in separate PRs).

It also depends on gtk2.

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