chore(editor): remove deprecated style fixes

[hejtful]

• fa-4x class doesn't seem to be used anymore
• the chromium bug seems to have been resolved by one of the Slate updates

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Name	Status	Preview	Comments	Updated (UTC)
serlo-editor-for-edusharing	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Mar 18, 2024 1:28pm