Remove `mate-media-gstreamer`.

Hi,

It is no longer possible to build `mate-media` on Arch Linux using the gstreamer 0.10.x backend. This reasons are documented here:

  * <https://github.com/mate-desktop/mate-media/issues/28>

When MATE stable migrates to Gstreamer 1.0 then I will package `mate-media` for both the Pulse Audio and Gstreamer backends.

Regards, Martin.