remove manual git completion

Previously I used (an outdated) .git-completion.sh from git.git, which
is not designed for zsh, but rather bash. Zsh actually provides a
default zsh-friendly git completion, so at this time, I’ll simply use
that.

Even if I want to switch back to the git.git completion in the future,
there’s no need to configure the git completion manually. Oh-my-zsh has
a plugin named `gitfast` that provides the functionality (it is said
that git.git completion is faster and more robust than the zsh default;
let’s see).