pcmcia: remove Motorola MBX860 support

The CONFIG_MBX symbol is not defined anywhere in the kernel tree, which means
this platform is not supported by the Linux kernel and we can remove the
corresponding code from the 'm8xx_pcmcia' driver.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Cc: linux-pcmcia@lists.infradead.org
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: linux-m68k@lists.linux-m68k.org
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>