btrfs: remove duplicated include in block-group.c

disk-io.h is included more than once in block-group.c, remove it.
Reviewed-by: NJohannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: NTiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/block-group.c

@@ -7,7 +7,6 @@
 #include "disk-io.h"
 #include "free-space-cache.h"
 #include "free-space-tree.h"
-#include "disk-io.h"
 #include "volumes.h"
 #include "transaction.h"
 #include "ref-verify.h"