remove the special casing for res->first in VP8SetResidualCoeffs

if res->first = 1, coeffs[0]=0 because of quant.c:749 and line
added at quant.c:744
So, no need for the extra case.
Going forward, TrellisQuantizeBlock() should also be calling
a variant of VP8SetResidualCoeffs() to set the 'last' field.

also: fixes a warning for win64
    + slight speed-up

Change-Id: Ib24b611f7396d24aeb5b56dc74d5c39160f048f0