Implement "Unnecessary parent reference" rule

Need to implement a lint rule that reports unnecessary parent references:

```scss
p {
  & > .foo {
    ...
  }
}
```

This is similar to scss-lint's [UnnecessaryParentReference](https://github.com/brigade/scss-lint/blob/master/lib/scss_lint/linter/README.md#unnecessaryparentreference).