[EuiIcon] Rewrite to a functional component

[tkajtoch]

Summary

As part of the EuiIcon refactor initiative, we want to rewrite it to a functional component. This will simplify maintenance and help eliminate unused fragments of code.

Acceptance criteria

• The component should be rewritten to a functional component
• enqueueStateChange should be removed as it has no positive effect in modern versions of React
• Icon cache logic should be left as-is; it'll be addressed in a separate issue