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>