Currently, when new-multisymbol and ext-comp-refs are
both enabled, the comp_ref_type_prob and uni_comp_ref_prob
arrays are forward updated in the compressed header, but
their values are never used.

We can save a tiny bit of space by not signalling the
forward updates at all; as a side benefit, we can wrap
the whole of read_frame_reference_mode_probs() in
a #if !CONFIG_NEW_MULTISYMBOL block.

Note: It seems the encoder already does this; I think
the reason it doesn't cause mismatches is because we
effectively pad the compressed header out to a full byte,
and av1_diff_update_prob() reads a bit with a very high
probability of being 0.

Change-Id: Ia1fca05ee498609c98c7ea1f89578f6783357d7f