xfs: Remove duplicated include in xfs_super

Fix following checkincludes.pl warning:
./fs/xfs/xfs_super.c: xfs_btree.h is included more than once.

The include is in line 15. Remove the duplicated here.
Signed-off-by: Wan Jiabing <wanjiabing@vivo.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>

fs/xfs/xfs_super.c

@@ -37,7 +37,6 @@
 #include "xfs_reflink.h"
 #include "xfs_pwork.h"
 #include "xfs_ag.h"
-#include "xfs_btree.h"
 #include "xfs_defer.h"
 
 #include <linux/magic.h>