remove obsolete pywikibot.stopme() at the end of this 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: I1dcb7930a47c9c58f867c1710d3b4f4600341dd7
Merge "remove obsolete pywikibot.stopme() at the end of this script."