Before this, this function was copied-and-edited from
av1_build_inter_predictors_sby() and av1_build_inter_predictors_sbuv().
Also, some places used sb() one directly (like rdopt.c), while others used
sby() and sbuv() separately (like encodeframe.c --> encode_superblock()).

This was prone to errors, if someone mistakenly modified only one/two of
the 3 functions.

Now, we just reuse sby() and sbuv() functions in sb().

Verified that there's no change in output.

Change-Id: If81d78e767cc3fa09f06094d4db36ff1eaea3e4f