USB: Remove test for NULL that'll never happen in usb_disconnect()

In drivers/usb/core/hub.c::usb_disconnect(), 'udev' will never be
NULL, so remove the test and printing of debug message.

Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>