xfs: Remove empty declartion in header file

commit f6225eeb upstream.

The definition of xfs_attr_use_log_assist() has been removed since
commit d9c61ccb ("xfs: move xfs_attr_use_log_assist out of xfs_log.c").
So, Remove the empty declartion in header files.

Signed-off-by: Zhang Zekun <zhangzekun11@huawei.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Carlos Maiolino <cem@kernel.org>
Signed-off-by: Catherine Hoang <catherine.hoang@oracle.com>
Acked-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>