Remove obsolete git-branch recipe

The git-branch recipe says: »Note that the next release will probably
allow repo.listall_branches().« Concluding from the README,
Repository.listall_branches() was included in some release prior to
0.20.0, so at least that statement is obsolete.

However, since pygit2.org brings up fairly accurate results for a search
on »list all branches«, I figured that the whole recipe isn't needed
anymore. Therefore delete it.

Author: rmoehn

docs/recipes.rst

@@ -17,7 +17,6 @@ Main porcelain commands
 .. toctree::
    :maxdepth: 1
 
-   git-branch (List, create, or delete branches.) <recipes/git-branch>
    git-init (Create an empty git repository or reinitialize an existing one.) <recipes/git-init>
    git-log (Show commit logs.) <recipes/git-log>
    git-show (Show various types of objects.) <recipes/git-show>