sh64: Kill off unused fixed I/O mapping window.

This was reworked some time ago to go through fixmaps instead, leaving
the range itself unused. As such, kill off the remaining references and
hand over the remaining space for fixmaps directly. This also makes it
possible to simplify the vmalloc fault case as we no longer have to care
about the special section.

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