Remove 'guard-for-in' lint rule (#1773)

Iterating over an object's keys using `for/in` is idiomatic and it's safe (in all modern browsers) to not check hasOwnProperty as long as the object is a plain object. Can we remove this lint rule?

packages/eslint-config-react-app/index.js

@@ -61,7 +61,6 @@ module.exports = {
     'default-case': ['warn', { commentPattern: '^no default$' }],
     'dot-location': ['warn', 'property'],
     eqeqeq: ['warn', 'allow-null'],
-    'guard-for-in': 'warn',
     'new-parens': 'warn',
     'no-array-constructor': 'warn',
     'no-caller': 'warn',