remove sad x3,x8 specializations

remove sad x3,x8 specializations

These would compute the sum of absolute differences (sad) for a
group of 3 or 8 references. This was used as part of an exhaustive
search.

vp8 only uses these functions in speed 0 and best quality.

For vp9 this is only used with the --enable-non-greedy-mv
experiment.

This removes the 3- and 8-at-a-time optimized functions and uses
the fall back code which will process 1 or 4 (vpx_sadMxNx4d) at
a time.

For configure --target=x86_64-linux-gcc --enable-realtime-only:
libvpx.a
before: 3002424 after: 2937622 delta: 64802
after 'strip libvpx.a'
before: 2116998 after: 2073090 delta: 43908

Change-Id: I566d06e027c327b3bede68649dd551bba81a848e