Remove Skins Restorer integration

[pdinklag]

The Skins Restorer API is an unreliable mess that seems to be subject to unnecessary breaking changes and doesn't seem to work properly in various circumstances for unclear reasons (see #254 , #245, #240). So far, it has caused me more headache than it has really contributed to MinecraftStats and I don't see why I should fix here what's broken there - especially since whatever I do today may no longer work tomorrow.

To quote the SkinsRestorer API website:

Please note that this API is still work-in-progress. Expect breaking changes!

Until the Skins Restorer API works properly and reliably, the integration will be removed because I cannot maintain it.

The fact that it now requires Java 17 or later poses another issue that I will have to deal with at another time.