usb: gadget: f_fs: drop duplicate usb_functionfs_descs_head declaration

usb: gadget: f_fs: drop duplicate usb_functionfs_descs_head declaration

Applicable for 3.14-stable only, as this revertes a previous commit that was incorrect.

This commit drops duplicate declaration of struct usb_functionfs_descs_head
erronousely added in commit 28c5980b54 ("usb: gadget: f_fs: resurect
usb_functionfs_descs_head structure").

Fix from 28c5980b54 is applicable only for v3.15-rc1 and newer kernels.

This fixes error in uapi:

  /src/linux$ make -C tools usb

  make: Entering directory '/src/linux/tools'
    DESCEND  usb
  make[1]: Entering directory '/src/linux/tools/usb'
  gcc -Wall -Wextra -g -I../include -o testusb testusb.c -lpthread
  gcc -Wall -Wextra -g -I../include -o ffs-test ffs-test.c -lpthread
  In file included from ffs-test.c:41:0:
  ../../include/uapi/linux/usb/functionfs.h:42:8: error: redefinition of ‘struct usb_functionfs_descs_head’
   struct usb_functionfs_descs_head {
          ^
  ../../include/uapi/linux/usb/functionfs.h:31:8: note: originally defined here
   struct usb_functionfs_descs_head {
          ^

Cc: Michal Nazarewicz <mina86@mina86.com>
Cc: Felipe Balbi <balbi@ti.com>
Signed-off-by: Karol Lewandowski <k.lewandowsk@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>