Remove root font-size 62.5%

[ronilaukkarinen]

A good point brought up by latenssi on wpfi Slack: Should we even have 62,5% nowadays? Article: Should I change the default HTML font-size to 62.5%?. Because we use it in :root rather than html, it's not a problem, but it's not needed any more because we have a stylelint plugin that converts the rems automatically.

We should perhaps revert back to the default soon.