usb: gadget: file_storage: remove its last pieces

This patch removes the last pieces of the file_storage gadget hidden in
storage_common behind __maybe_unused bars.
The CBI bits have no user on the gadget side. Only file_storage
implemented the obsolete protocol.
The additional USB3.0 descriptors were served by file_storage, the other
gadgets are using composite for this.

Acked-by: Michal Nazarewicz <mpn@google.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>