usb: gadget: f_hid: remove unnecessary usb_ep_dequeue()

Function usb_ep_disable() causes completion of all requests queued
for given endpoint, so there is no need to dequeue them after endpoint
disabling.

Signed-off-by: Robert Baldyga <r.baldyga@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>