Remove Skins Restorer integration #255

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.