Remove obsolete Animated Raster Graphics (ARG) support

Reasons for removal:

 * There have been no substantial changes since forking PIL.
 * Opening an image gives this warning:

>  "The ArgImagePlugin driver is obsolete, and will be removed from a future release of PIL.  If you rely on this module, please contact the PIL authors."

 * It's now the future. Has anyone made contact about relying on it? 
 * I can't find much information about the ARG image format except for this [1996 proposal](http://www.datapacrat.com/True/MPNG/ARG-1-19.TXT) (from PIL author) Fredrik Lundh and as a competing proposal to a [1998 MNG and JNG proposal
(ftp://ftp.simplesystems.org/pub/libpng/mng/documents/history/MNG-status-19981111.html):

> Competing proposals
> Two competing proposals have also been written, both around January 1997. The drafts for these proposals can be found at the MNG ftp site mentioned above. They are
> * "ARG: Animated Raster Graphics"
> * "DOH: Dancing Optical Hallucinations" (this is a temporary name; if this proposal is accepted in lieu of MNG, it will probably be called MNG).

 * It looks like ARG never made it beyond a proposal. If no-one uses it we don't need to keep the code. Removing it also increases coverage (see https://github.com/python-pillow/Pillow/issues/722).