Removed source download links from the navigation bar.

I tried committing both binary downloads into gh-pages so that they could be downloaded from http://cspotcode.github.io/Gmk-Splitter/.  However, GitHub hasn't updated the page with my latest commits to gh-pages.  Maybe you are more correct than me, and GitHub is refusing to host large downloads.

This three-line pull request removes the green ZIP and TAR download buttons from the automatically-generated, because I had promised to send you a patch yesterday.