This was a side-effect of an earlier change that was done to run
end-callbacks before terminating external processes (as those callbacks
may be used, for example, to remove temporary files).
Many of interpreters had logic in those callbacks to reset debugger.pid,
which was useful (at a time) to reset UI after terminatin of a process
(introduced by 4554c67c). However, this logic was unnecesary in
interpreters and interfered with terminating running processes, so this
commit removes it from all the interpreters and does in the IDE itself.
