Add `--remove-blocks` option to `pin rm` and `pin update` APIs

A user in a public room, jwh (convo starts [here]( https://matrix.to/#/!nxEltmLkHUANNChiSF:matrix.thedisco.zone/$7yctZs15hgDsYS2OI152qQ7dU0KLM5VluDrY03AIG0E?via=matrix.thedisco.zone&via=matrix.org&via=asra.gr)) suggested that removing pins, also removes blocks. Their use-case is they operate repositories, and they'd rather blocks get removed as pins get removed/updated as they don't need that data anymore. They also mentioned that sometimes users may accidentally attempt to publish sensitive data, and this feature would help users stop their node from broadcasting that information before it's too late.

I think this would be best as an optional, disabled by default flag `--remove-blocks` to the `pin rm` and `pin update` APIs. This would remove all the blocks that aren't referenced by another object as the pinned blocks are unpinned.

Open to thoughts / suggestions / opinions :). I couldn't find this as an issue already, sorry if it's already been discussed!