[Misc]: Remove `contrib` folder in repo

[tulpenkiste]

The contrib folder contains 3 seperate patch files, ranging from 8 years old to 16 years old. I highly doubt these diffs even work on modern revisions, nor do they serve a purpose.

• supertux-nogl - replacable via cmake (already done iirc)
• supertux-coop - already implemented
• schroedingers-boxes-pr-390 -

[swagtoy]

I support this when i was looking at them. However, I think we should do patches instead of maintaining our own Git submodules since we only do very minor things (and some of them unneeded nowadays)