remove magit-svn.el, magit-stgit.el, magit-topgit.el and bin/magit

Remove the libraries `magit-svn.el', `magit-stgit.el' and
`magit-topgit.el' as well as the shell-script `bin/magit'
and create a new separate repository for each one of them.

Also see https://github.com/magit/magit/pull/1496.
Merge branch 'liberate-extensions' into next