xfs: Remove empty declartion in header file

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>

fs/xfs/xfs_log.h

@@ -158,6 +158,4 @@ bool	xfs_log_check_lsn(struct xfs_mount *, xfs_lsn_t);
 
 bool	  xlog_force_shutdown(struct xlog *log, uint32_t shutdown_flags);
 
-int xfs_attr_use_log_assist(struct xfs_mount *mp);
-
 #endif	/* __XFS_LOG_H__ */