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: Ieb1f20cf862a7bcc263d3ce5b0c7c0c578046c5a
Merge "remove obsolete pywikibot.stopme() at the end of the script."