[cleanup] remove login.showCaptchaWindow() method
[cleanup] remove login.showCaptchaWindow() method

login.showCaptchaWindow() was never implemented and does nothing.
Remove this senseless method then after 12 years.

Change-Id: Id7af3b9eea403f1c5d55619f75a0a8264ebe8667

wikimedia/pywikibot-core,wikimedia/pywikibot-core,PersianWikipedia/pywikibot-core