sh: Kill off broken direct-mapped cache mode.

Forcing direct-mapped worked on certain older 2-way set associative
parts, but was always error prone on 4-way parts. As these are the
norm these days, there is not much point in continuing to support this
mode. Most of the folks that used direct-mapped mode generally just
wanted writethrough caching in the first place..

Signed-off-by: Paul Mundt <lethal@linux-sh.org>