Simplify ESLint configuration

[kripod]

The following parts of .eslintrc.json should be removed on the long term:

"settings": {
  "import/core-modules": ["gatsby"]
}

Related issue: gatsbyjs/gatsby#5410

---

"rules": {
  "jsx-a11y/anchor-is-valid": [
    "error",
    {
      "components": ["Link"],
      "specialLink": ["to"],
      "aspects": ["noHref", "invalidHref", "preferButton"]
    }
  ]
}

Related PR: airbnb/javascript#1648