sh: Kill off the now unused ARCH_USES_GETTIMEOFFSET code.

Now that the stragglers (MTU2/CMT/etc.) have been rewritten and we are
selecting both GENERIC_TIME and GENERIC_CLOCKEVENTS, the get_offset()
timer op is completely unused. As a result, we are now able to kill off
the ARCH_USES_GETTIMEOFFSET references.

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