Remove merged dependencies from magit-pkg.el

Hi,

When attempting to install the next branch using Cask, like so:

```(depends-on "magit" :git "https://github.com/magit/magit.git" :branch "next")```

I get the following error:

```Dependency magit failed to install: Package `git-commit-1.0.0' is unavailable```

Since these dependencies are now in the main repo, I don't think they need to specified as package dependencies? I can confirm that installing using the patched branch works, and that `git-commit-mode` and `git-rebase-mode` are both available and working.