Removing vpx_codec_vp9x_cx and internal experimental flag.

vpx_codec_vp9x_cx is not used internally. Experimental flag from
vp9_extracfg is also not really used. YUV 4:4:4 just works after these
changes (you have to specify --profile=1 for the encoder).

Change-Id: Ib1c8461d0d19d159827e005efe868f891eea0140