bug#21721: update blurb on http://savannah.gnu.org/projects/emacs/

[Eli Zaretskii]

> From: Glenn Morris <rgm@gnu.org>
> Date: Tue, 20 Oct 2015 15:35:38 -0400
> 
> Please update the blurb on http://savannah.gnu.org/projects/emacs/
> about emacs-24. I suggest simply removing all the following:
> 
>     or
> 
>     git clone -b emacs-24 git://git.sv.gnu.org/emacs.git
> 
>     Where "emacs-24" is the recommended branch if you want to help us
>     test&debug the code before the next release. "master" is if you're
>     actively working on Emacs's own code.

Done.