sh: Kill off sh_bios_in_gdb_mode().

With the reworked kgdb support, we always detach and reinitialize the
stub. This was mostly a feature for handoffs between sh-ipl+g and the
kgdb stub, but virtually no sh-ipl+g versions ever had this working
right in the first place.

Given that the sh-ipl+g stubs in general use today don't even support
the GDB stub, and we have already killed off the special casing in the
sh-sci serial driver, kill off this now unused symbol too.

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