Removed functions:
* vp9_mse16x16_mmx
* vp9_get_mb_ss_mmx
* vp9_get4x4var_mmx
* vp9_get8x8var_mmx
* vp9_variance4x4_mmx
* vp9_variance8x8_mmx
* vp9_variance16x16_mmx
* vp9_variance16x8_mmx
* vp9_variance8x16_mmx

They all have SSE2 equivalent.

Change-Id: I3796f2477c4f59b35b4828f46a300c16e62a2615