btrfs: remove unused definition of tree_entry in extent-io-tree.c

The declaration was temporarily moved in a4055213 ("btrfs: unexport
all the temporary exports for extent-io-tree.c") and then should have
been removed in 6.0 in 071d19f5 ("btrfs: remove struct tree_entry in
extent-io-tree.c") but was not.  This was found by tool
https://github.com/jirislaby/clang-struct .
Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent-io-tree.c

@@ -87,12 +87,6 @@ static inline void __btrfs_debug_check_extent_io_range(const char *caller,
  */
 static struct lock_class_key file_extent_tree_class;
 
-struct tree_entry {
-	u64 start;
-	u64 end;
-	struct rb_node rb_node;
-};
-
 void extent_io_tree_init(struct btrfs_fs_info *fs_info,
 			 struct extent_io_tree *tree, unsigned int owner)
 {