Remove the slots from AST nodes that was meant to point to its parent.
Remove the slots from AST nodes that was meant to point to its parent.

Originally it was believed it would be useful, but it has never been
used, and was always 0.  Which is a good thing, since the dtor was
trying to free the parent too :)

	* src/ast/ast.yml (Ast::up): Remove.

urbiforge/urbi,aldebaran/urbi,aldebaran/urbi,aldebaran/urbi,urbiforge/urbi,aldebaran/urbi,aldebaran/urbi,urbiforge/urbi,urbiforge/urbi,urbiforge/urbi,urbiforge/urbi,urbiforge/urbi,urbiforge/urbi,aldebaran/urbi,aldebaran/urbi,aldebaran/urbi,urbiforge/urbi