Remove Wextra warnings from vp9_sad.c

Remove Wextra warnings from vp9_sad.c

As a side-effect, the max_sad check is removed from the
C-implementation of VP8, for consistency with VP9, and to
ensure that the SAD tests common to VP8/VP9 pass.
That will make the VP8 C implementation of sad a little slower
but given that is rarely used in practice, the impact will be
minimal.

Change-Id: I7f43089fdea047fbf1862e40c21e4715c30f07ca