remove parser context field from cfg_obj_t

remove parser context field from cfg_obj_t

cfg_obj_t doesn't store a pointer to its a parser context anymore,
and does not depend on the parser's lifecycle. Instead, it stores a
reference to its own memory context (and in principle, each node
could have different memory context). This also slightly simplifies
the _destroy API as there is no need to pass a context through it
anymore.