USB: remove staging quatech_usb2 driver

Now that we have a "real" driver for the quatech devices, we can remove
the drivers/staging/quatech_usb2/ driver as it is no longer needed.

Thanks to Bill Pemberton for writing the new driver.

Cc: Richard Ash <richard@audacityteam.org>
Cc: Bill Pemberton <wfp5p@virginia.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>