btrfs: remove extent_map_tree forward declaration at extent_io.h

There's no need to do a forward declaration of struct extent_map_tree at
extent_io.h, as there are no function prototypes, inline functions or data
structures that refer to struct extent_map_tree.

So remove that forward declaration, which is not needed since commit
477a30ba ("btrfs: Sink extent_tree arguments in
try_release_extent_mapping").
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent_io.h

@@ -215,8 +215,6 @@ static inline void extent_changeset_free(struct extent_changeset *changeset)
 	kfree(changeset);
 }
 
-struct extent_map_tree;
-
 int try_release_extent_mapping(struct page *page, gfp_t mask);
 int try_release_extent_buffer(struct page *page);