delete lesser links, keep the good ones

I'm going to send this through a PR like a good GitHub citizen rather than pushing directly to the repo like I usually do, because I feel like maybe everyone's not onboard with this.

I'd rather the list of Git resources be a compilation of the very best than a comprehensive set of links. As such, I deleted a bunch which are OK but imperfect, leaving what I consider the best tutorials and resources on the list. If you disagree, please chime up in the comments.