remove unused "global" declaration; remove obsolete stopme()

bot object is never used as global variable. The global
declaration is obsolete.

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: I8d7c07cd058f6c7db236c84ab461489b5fbfc088
Merge "remove unused "global" declaration; remove obsolete stopme()"