remove obsolete pywikibot.stopme() at the end of the script.

In core branch pywikibot.stopme() is called by atexit library.
The function is executed upon normal program termination. This
patch prohibits executing it twice.

Change-Id: I9171ba44407ec1848c29e5ea69482d83b1e457ff
Merge "remove obsolete pywikibot.stopme() at the end of the script."