Converted react-unused-props-and-state to use a walk function.

[reduckted]

PR checklist

• Addresses an existing issue: Convert all existing rules to use a walk function instead of extending AbstractRule #680
• New feature, bugfix, or enhancement
  • Includes tests
• Documentation update

Overview of change:

Converted react-unused-props-and-state to use a walk function.

Is there anything you'd like reviewers to focus on?

This one's a bit odd because it sort of does two passes over the AST. It walks everything once, but doesn't step down on ClassDeclaration nodes. Then it does a "second pass" by stepping down on the ClassDeclaration nodes it saw in the first pass. I considered splitting it up into two separate callback functions, but I wasn't sure exactly what's expected to be visited on the first pass and what's visited on the second pass, so I just used one callback function which is the equivalent of what the walker class did.

[JoshuaKGoldberg]

Thanks for the notes - seems fine the way you have it. This is kind of a spooky rule.