'internal' modifier is redundant for static method declared in an internal extension

Encoder.swift

```
internal extension DecodingError {
    internal static func _typeMismatch(at path: [CodingKey], expectation: Any.Type, reality: Any) -> DecodingError { ...
```

Xcode 11.4 shows warning that the internal isn't required. Not sure what backwards compatibility is required but could be removed for future releasese