Remove skip_txfm optimization.

Commit 0d6980d7 removed some use
of the skip_txfm optimization, and the rest are not productive.

The current use of this optimization is only used with --good
and --cpu-used >= 3, however the overhead of this is higher than the
speedup it yields.

Removing this, and subsequently simplifying model_rd_for_sb yields
a net encoder speedup:
--cpu-used=0    ~1.5% faster
--cpu-used=3    ~2.0% faster

The code simplification is also significant.

Change-Id: I1dd668c32de15a2e912c59c42379d0f9e1032ff8