Unneeded raw String

[asashour]

For a string without special characters, a raw string is not needed, e.g.

  var x = r'abc';
  var y = r'''
something
''';

I think it is a little distracting to have a raw prefix, while the string is completely fine without it.

[bwilkerson]

I believe that this is covered by unnecessary_raw_strings. Let us know if that doesn't perform as expected.