[MIPS] Remove superfluous "ifdef CONFIG_KGDB".

Given that the Makefiles involved already have conditional compilation of
 the form:

  obj-$(CONFIG_KGDB)      += dbg_io.o

there seems to be little value for the dbg_io.c source files to check
that config variable yet again.

Signed-off-by: Robert P. J. Day <rpjday@mindspring.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>