Remove legacy context-based debugging in favor of CodeContext (#866)

Remove legacy context-based debugging in favor of CodeContext (#866)

Added in 1998, context-based debugging has been neglected and

- only covers two relatively small contexts
- unsafe in the presence of exceptions
- produces noise (e.g., "ctx: exit level 0" messages)
- delayed "ctx: exit" messages confuse admins
- uses deprecated urlXXX() API
- difficult to extend to more contexts without performance overheads
- usually provides less info in fewer contexts (than CodeContext)