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: I0489098e896f307e9e22c4ef45f1488b3cfda97a
Merge "remove obsolete pywikibot.stopme() at the end of the script."