Add further missing delete_callbacks_for_context.

Having explicitly _stated_ in commit 4dcc0fddf the principle that if
you ever queue a toplevel callback on a freeable object then you
should also call delete_callbacks_for_context on that object before
freeing it, I realised I'd never actually gone through and checked
methodically at every call site of queue_toplevel_callback. So I did,
and naturally, I found several missing ones.

terminal.c

@@ -1804,6 +1804,7 @@ void term_free(Terminal *term)
     sfree(term->tabs);
 
     expire_timer_context(term);
+    delete_callbacks_for_context(term);
 
     conf_free(term->conf);