EXPORT_UNUSED_SYMBOL* is not actually used anywhere.  Remove the
unused functionality as we generally just remove unused code anyway.
Reviewed-by: NMiroslav Benes <mbenes@suse.cz>
Reviewed-by: NEmil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NJessica Yu <jeyu@kernel.org>