Remove unused --color-severe

[wazawoo]

🧭 Context

• I wanted to test all alert types to see their colors on my site
• looking at colors in css, I saw --color-severe so I assumed there was an alert type for severe
• seeing that there wasn't, I wanted to remove the unused color to reduce confusion for anyone else
• if severe should be here, I can instead add it, but an icon would be needed and I'm not sure what's expected there

📚 Description

• remove --color-severe from css files as it is referenced nowhere else