xfs: kill XFS_BTREE_MAXLEVELS

Nobody uses this symbol anymore, so kill it.

Signed-off-by: Darrick J. Wong <[e-mail]>
Reviewed-by: Chandan Babu R <[e-mail]>
Reviewed-by: Dave Chinner <[e-mail]>

fs/xfs/libxfs/xfs_btree.h

···
 #define XFS_BTREE_STATS_ADD(cur, stat, val)	\
 	XFS_STATS_ADD_OFF((cur)->bc_mp, (cur)->bc_statoff + __XBTS_ ## stat, val)
 
-#define	XFS_BTREE_MAXLEVELS	9	/* max of all btrees */
-
 /*
  * The btree cursor zone hands out cursors that can handle up to this many
  * levels.  This is the known maximum for all btree types.