btrfs: remove duplicate includes

These duplicate includes have been found with scripts/checkincludes.pl but
they have been removed manually to avoid removing false positives.
Signed-off-by: Pravin Shedge <pravin.shedge4linux@gmail.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/super.c

@@ -61,7 +61,6 @@
 #include "tests/btrfs-tests.h"
 
 #include "qgroup.h"
-#include "backref.h"
 #define CREATE_TRACE_POINTS
 #include <trace/events/btrfs.h>