usb: gadget: mass_storage: remove unused options

This commit removes thread_name and lun_name_format fields from the
fsg_config structure.  Those fields are not used by any in-tree code
and their usefulness is rather theoretical.

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