usb: gadget: Remove File-backed Storage Gadget (g_file_storage).

The File-backed Storage Gadget (g_file_storage) gadget has been replaced
with Mass Storage Gadget (g_mass_storage) which uses the composite
framework.  This commit removes g_file_storage (and most references to it).

Signed-off-by: Michal Nazarewicz <mina86@mina86.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Felipe Balbi <balbi@ti.com>