vp9: Remove assert from model_rd in non-rd pickmode.
vp9: Remove assert from model_rd in non-rd pickmode.

The assert checks for tx_size >= 8x8, but 4x4 can
be set in some cases.

Change-Id: I8bf9683e1add768becaa1208e1709ad0470e3850

ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx