scripts/revertbot.py: Remove RevertBot class
scripts/revertbot.py: Remove RevertBot class

`RevertBot` has been intented as a sample code. Using it as the main bot
class does not work as expected (T194909). Move `RevertBot` to module's
docstring. Use `BaseRevertBot` instead of `RevertBot`.

Bug: T194909
Change-Id: Ie094c2c524ae2aaf1390aa63ef207f067d3a0f7c

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