Remove wagtailpolls

https://github.com/neon-jungle/wagtailpolls show it's been inactive
for 5 years.

README.md

@@ -154,7 +154,6 @@ Awesome Wagtail [![Awesome](https://cdn.rawgit.com/sindresorhus/awesome/d7305f38
 
 ### Forms
 
-- [wagtailpolls](https://github.com/takeflight/wagtailpolls) - A plugin for adding polling capabilities to the Wagtail CMS.
 - [Wagtailsurveys](https://github.com/torchbox/wagtailsurveys) - A module for Wagtail which provides the ability to build polls and surveys.
 - [Wagtail ReCaptcha](https://github.com/springload/wagtail-django-recaptcha) - wagtail-django-captcha provides an easy way to integrate the [django-recaptcha](https://github.com/praekelt/django-recaptcha) field when using the Wagtail formbuilder.
 - [Wagtail Simple Captcha](https://github.com/acarasimon96/wagtail-django-simple-captcha) - A self-hosted alternative to Wagtail ReCaptcha that easily integrates a [django-simple-captcha](https://github.com/mbi/django-simple-captcha) field into the Wagtail form builder.