staging: mt7621-mmc: Remove unused macro is_card_present

The macro is_card_present is unused and also a duplicate of a macro in
mmc, so remove it.

Signed-off-by: Christian Lütke-Stetzkamp <christian@lkamp.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>