Remove unused method BlankSource supportsParserType()

Problem/Motivation

BlankSource.php contains a supportsParserType() method:

  /**
   * {@inheritdoc}
   */
  public function supportsParserType($parser_type) {
    // All parsers are supported by this custom source type.
    return TRUE;
  }

However, this is never called anywhere, and it is not part of the extended class CustomSourceBase. I only found it because I was debugging a class that extends it (JsonSource.php).

Therefore, I propose that it be removed.