staging: exfat: Remove FAT/VFAT mount support, part 1

Remove the top-level mount functionality, to make this driver handle
only exfat file systems.

Signed-off-by: Valdis Kletnieks <Valdis.Kletnieks@vt.edu>
Link: https://lore.kernel.org/r/20191112211238.156490-2-Valdis.Kletnieks@vt.edu


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>