USB: storage: remove invalid URL from drivers

The old URL for usb-storage driver help is long gone.  So remove it from
the comments to not confuse people anymore.

Reported-by: Matthew Dharm <mdharm-usb@one-eyed-alien.net>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>