av1_update_mv_context() is only used to provide compound_mode_context,
which is the same as mode_context in find_mv_refs_idx(). This patch
removes the calling of av1_update_mv_context() that takes 0.5% of the
decoder time. This doesn't change bitstream.

Change-Id: I6f0e082b237ff42c3b3e72361c46f98249ba07ab