Duplicate function call when processing source path

Problem/Motivation

As a result of work done in #3448498: Public method Redirect::generateHash() should unify path before generating the hash, the path is converted to lowercase when generating the hash. That should be unnecessary.

public static function generateHash($source_path, array $source_query, $language) {
  // Remove leading and trailing slashes, and convert to lowercase.
  $source_path = trim(mb_strtolower($source_path), '/');
  $hash = [
    'source' => mb_strtolower($source_path),

Proposed resolution

Remove duplicate use of mb_strtolower