staging: mt7621-mmc: Remove unused sdio irq code

Currently the code for the sdio irq is never used, because the flags
for it (MSDC_(EXT_)SDIO_IRQ) are never set. So the whole code for it
can be removed.

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