staging: m57621-mmc: delete driver from the tree.

The license text in this driver is "interesting" and not really obvious
that it is supposed to be able to be distributed in the kernel source
tree.  Yes, the MODULE_LICENSE() text says GPL, so it's probably ok, but
to be safe, I am deleting this driver.  I will be glad to add it back if
the license is properly sorted out, but for now, this isn't worth the
potential risk, I should have never taken it in the first place.

Cc: Matthias Brugger <matthias.bgg@gmail.com>
Cc: NeilBrown <neil@brown.name>
Cc: George Hilliard <thirtythreeforty@gmail.com>
Cc: "Christian Lütke-Stetzkamp" <christian@lkamp.de>
Cc: Nishad Kamdar <nishadkamdar@gmail.com>
Cc: Sergej Perschin <ser.perschin@gmail.com>
Cc: John Crispin <blogic@openwrt.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>