staging: mt7621-mmc: Remove unused code from board.h

The board.h file contins lot of never used code, to clean up the
driver, this code is removed.

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