[REM] website(_blog/_event): remove filtercolor option on cover

addons/website/static/src/js/editor/snippets.options.js

addons/website/static/src/js/editor/wysiwyg_multizone.js

addons/website/views/snippets/snippets.xml

addons/website/views/website_templates.xml

addons/website_blog/data/website_blog_data.xml

addons/website_blog/data/website_blog_demo.xml

addons/website_blog/models/website_blog.py

addons/website_event/data/event_demo.xml

addons/website_event/models/event_event.py