USB: gadget: Remove configfs usage

Userspace after mounting configfs may continue to
use configfs if usb_gadget supports configfs. For this
userspace just relies on the presence of /config/usb_gadget.
As 3.18 kernel USB drivers don't full support configfs but
rather rely on android gadget, hence remove compilation of
usb configfs so that usb_gadget node doesnt appear in /config
and userspace can fallback to android gadget.

Change-Id: Ic59adfacd3972fd54608f916c98478525b9a9286
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>