Remove bogus disconnect test introduced by 95bef012 ("USB: more serial
drivers writing after disconnect") which prevented queued data from
being freed on disconnect.

The possible IO it was supposed to prevent is long gone.

Cc: stable <stable@vger.kernel.org>
Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>