Remove gitaly-remote command

Since the gitaly-remote command was added in 22eb30d2 (Add gitaly-remote binary to add or update remote URL, 2019-03-27), it didn't catch much interest and is not really used anywhere. It does require us to do quite some build hacks, though, because of its dependency on git2go and thus indirectly on libgit2, and in fact building gitaly-remote does not currently work. Furthermore, it breaks our gosec-sast job that performs scanning for security vulnerabilities.

So given that we'd require additional hacks to keep gitaly-remote while nobody really uses it, let's instead remove it and the git2go/libgit2 dependencies altogether.

Fixes #1706 (closed)