Fix simplification errors that result in warnings after removing panes

[jleibs]

• When removing a tile, it needs to be removed from its parents list of children
• When simplifying a tree that collapses the root, use the correct tile_id post-simplification.

This results in a few API changes:

• remove_recursively is moved from the Tiles container to the Tree container.
• simplify_tile_id is replaced by simplify_children_of_tile to avoid ambiguity where simplification would cause that tile to be simplified out of existence.