usb: gadget: f_mass_storage: remove deprecated fsg_add()

There are no in-tree fsg_add() users and it has been deprecated
since 2.6.35 [1dc90985: fsg_add() renamed to fsg_bind_config()] so
out-of-tree users had more then enough time to convert.  Removing.

Signed-off-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>