staging: phison: remove driver

The driver hasn't been cleaned up and it doesn't look like anyone is
working on it anymore (including the original author). So remove the
driver from the kernel. If someone wants to work on cleaning it up and
moving it out of staging, this commit can be reverted.

Signed-off-by: Kristina Martšenko <kristina.martsenko@gmail.com>
Cc: Evan Ko <evan_ko@phison.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>