Remove all references to JobActivity

The JobActivity event got replaced by callback functions provided to
jobstart() or termopen().

It got removed here:

6e7757ad51

src/nvim/terminal.c

@@ -894,8 +894,7 @@ static void on_refresh(Event event)
   }
   Terminal *term;
   void *stub; (void)(stub);
-  // dont process autocommands while updating terminal buffers. JobActivity can
-  // be used act on terminal output.
+  // don't process autocommands while updating terminal buffers
   block_autocmds();
   map_foreach(invalidated_terminals, term, stub, {
     // TODO(SplinterOfChaos): Find the condition that makes term->buf invalid.