btrfs: remove duplicate include header in extent-tree.c

extent-tree.h is included more than once, added in a0231804 ("btrfs:
move extent-tree helpers into their own header file").
Signed-off-by: ye xingchen <ye.xingchen@zte.com.cn>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent-tree.c

@@ -38,7 +38,6 @@
 #include "dev-replace.h"
 #include "fs.h"
 #include "accessors.h"
-#include "extent-tree.h"
 #include "root-tree.h"
 #include "file-item.h"
 #include "orphan.h"