mmc: remove unused 'mode' from the mmc_host structure

This field and corresponding defines are simply never used anywhere
in the code.  But its mere presence is enough to confuse some host
driver authors who attempt to rely on it.  Let's eliminate the
possibility for confusion and remove it entirely.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>