cleanup: remove old, unused css

[marcelgerber]

I went through owid.scss a bit and removed some rules that are obviously not in use anymore.
For all of these, I ran ripgrep through live-data/bakedSite to see that they are actually not in use anymore.
The removed styles are certainly not an exhaustive list of the unused css rules, but it's a start.

Rules that I could identify:

• .owid-data used to be an "All our articles" block on the old homepage, circa 2018

  Details

  

• Fancybox used to be a jQuery-based image viewing tool that we certainly don't use any more.

• The .social component was this old one here, circa 2018

  Details

  

• #un-button I couldn't really find anywhere.

• .citation-note is the [cite] in the screenshot below, and .citation-guideline is the highlighted component here:

  Details

  

• .twitter-tweet is presumably an embedded tweet, of which we don't have any.

• .archive-pagination I cannot find anywhere 🤷🏻

• The teaching hub page neither contains #category-nav nor #entries-nav.

• The teaching hub h2 rules are completely overwritten by other, more specific rules.

• .mobile.subheading and .owid-presentations used to be features on the frontpage, the latter one being similar to .owid-data.

• Couldn't find .footer-widgets.

[mlbrgl]

thanks Marcel! The approach sounds solid.

I did a quick visual check on http://staging-site-remove-unused-css and everything looks ok. FWIW, I also haven't encountered any of these styles recently.

Some possibly related background on the embed tweet: #3149, slack