[IMPR] Derive UnusedFilesBot from other bot classes and simplify code
[IMPR] Derive UnusedFilesBot from other bot classes and simplify code

Derive UnusedFilesBot from SingleSiteBot, AutomaticTWSummaryBot
and ExistingPageBot

Change-Id: I063d1022da73414f53635eea53aecd229ad380d0

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