This code is unused in vp9. Only vp8 still contains references to
vpx_sad_NxMx[3|8] and only for sizes 16x16, 16x8, 8x16, 8x8 and 4x4.

Remove the remaining sizes and all the highbitdepth versions.

BUG=webm:1425

Change-Id: If6a253977c8e0c04599e25cbeb45f71a94f563e8