usb: gadget: langwell: don't call gadget's disconnect()

UDC core will call disconnect() and unbind() for us upon the gadget
removal, so we should not do it ourselves. Otherwise, a composite
gadget will explode, for example. Others might too.

This was introduced during conversion to new style gadget in 2c7f0989


(usb: gadget: langwell: convert to new style).

Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: stable@vger.kernel.org # v3.2
Cc: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Cc: Greg Kroah-Hartman <gregkh@suse.de>
Cc: linux-usb@vger.kernel.org
Signed-off-by: Felipe Balbi <balbi@ti.com>