The variation in boost calculation for gf and arf groups
is not significant enough to justify the extra complexity.
Also removed some other spurious code that no longer
has much material impact.

The handling of the rare case, where the boost bits
number is less than the number of bits a that would
be allocated if a frame was not boosted, will be dealt
with in a subsequent patch.

This change actually helps on all sets a little by
~0.1% - 0.2% with slightly bigger gains on SSIM.

Change-Id: Id42c1ac22a80a8c4993cfa0e51bc733eb9ed4f75