Twenty Nineteen: Remove duplicate `font-size` property.

The theme does not use a `px` fallback for `rem` units anywhere else.

Follow-up to [47053] for Twenty Twenty.

See #49015.
Built from https://develop.svn.wordpress.org/trunk@47140


git-svn-id: https://core.svn.wordpress.org/trunk@46940 1a063a9b-81f0-0310-95a4-ce76da25c4cd