vpx_dsp: clean up rtcd
vpx_dsp: clean up rtcd

Remove avx2+ssse3 specialization. Disabling ssse3 now automatically
disables avx2.

Change-Id: Id33eb6c85d1c4ee57128ebe45c995eb15cfcc765

webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,mwgoldsmith/vpx,webmproject/libvpx,Topopiccione/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx,mwgoldsmith/vpx,Topopiccione/libvpx,mwgoldsmith/libvpx,mwgoldsmith/vpx,Topopiccione/libvpx,mwgoldsmith/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,mwgoldsmith/vpx,mwgoldsmith/vpx,ShiftMediaProject/libvpx,mwgoldsmith/libvpx,mwgoldsmith/vpx,mwgoldsmith/libvpx,Topopiccione/libvpx,ShiftMediaProject/libvpx,Topopiccione/libvpx,mwgoldsmith/libvpx,mwgoldsmith/libvpx,Topopiccione/libvpx