Delete package.

[xaav]

Maintainers should be able to delete a package.

[markushausammann]

Actually in most cases, manual delete should not be needed, packagist should remove packages, when the respective tag has been deleted. Which is, as far as I can tell now, not the case.

[stof]

Versions are removed when they cannot be found (which is very fragile right now as a single failure is enough, leading to many issues when github fails like it does currently because of the DDoS attack).

However, a package itself cannot be removed, and this was already discussed a few times on IRC. The reasoning is that removing a package that existed previously will break all projects relying on it so it should ideally never be done. For this reason, the removal need to be done manually by @Seldaek for now.