usb: gadget: storage-common: Fix old comment about qualifier descriptor

Device qualifier descriptor is now generated by composite.c
code. So let's fix this old comment by removing parts which
are no longer valid.

Signed-off-by: Krzysztof Opasiak <k.opasiak@samsung.com>
Signed-off-by: Krzysztof Opasiak <kopasiak90@gmail.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>