[Tech debt] Remove `prop-types` as peer dependency

[cee-chen]

Per prop-types's own docs, they recommend libraries be a dependency and not a peer dependency: https://www.npmjs.com/package/prop-types#how-to-depend-on-this-package

It just seems odd to include it at this point (especially with our reliance on Typescript), so I would vote for removing it.

QA requirements

• We should do a prerelease and confirm against a create-react-app that a blank project with EUI still works without the peer dep
• We should also test Kibana locally against the prerelease RC to confirm that Kibana has no issues