vpx: remove compatibility defines
vpx: remove compatibility defines

We are guaranteed to have VPX_IMG_FMT_I420, VPX_PLANE_Y,
VPX_PLANE_U and VPX_PLANE_V as we require libvpx > 1.1.0.

https://bugzilla.gnome.org/show_bug.cgi?id=739476

ndufresne/gst-plugins-good,vatavuserban/gst-plugins-good,ndufresne/gst-plugins-good,wkatsak/gst-plugins-good,surround-io/gst-plugins-good,vatavuserban/gst-plugins-good,surround-io/gst-plugins-good,vatavuserban/gst-plugins-good,surround-io/gst-plugins-good,vatavuserban/gst-plugins-good,jpakkane/gstreamer-plugins-good,wkatsak/gst-plugins-good,ndufresne/gst-plugins-good,jpakkane/gstreamer-plugins-good,surround-io/gst-plugins-good,jpakkane/gstreamer-plugins-good,wkatsak/gst-plugins-good,wkatsak/gst-plugins-good,jpakkane/gstreamer-plugins-good,ndufresne/gst-plugins-good