USB: gadget: f_mass_storage: remove needless complete()

This commit removes call to the complete() function done in
fsg_unbind() which was never needed there but was a leftover
form file_storage.c.

Signed-off-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>