usb: gadget: storage_common: Remove FSG specific definitions.

Since g_file_storage has been removed, this commit removes code from
the storage_common.c file which has been used by file_storage.c only
(and not by f_mass_storage.c).

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