sh: Kill off last dead UBC header

Commit 7025bec9 ("sh: Kill off dead UBC
headers.") skipped arch/sh/include/cpu-sh2a/cpu/ubc.h. Since nothing is
using that header either, kill it off too.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>