Removed GIMP plugin

[2sh]

I should have put the GIMP plugin in a separate repository or such in the first place. I don't have time to maintain it anymore and I'd rather just focus on keeping the main stereoscopy repo up-to-date. The GIMP plugin probably would need a full rewrite as GIMP now uses Python3 and new functions and, as I understand it, gimpfu isn't and won't be supported by GIMP 3.x anymore without some hacky stuff.
But here is a gist if anyone wants to update it.