[9.x] Remove useless if statement in Str

[hAz5]

because min requirement of Laravel9 PHP version is 8 and above
we don't need to check this

  if (is_null($length) && PHP_MAJOR_VERSION < 8) {
      $length = mb_strlen($string, $encoding);
  }

[imanghafoori1]

Please explain why it is useless.

[crynobone]

Please explain why it is useless.

Laravel 9 doesn't support PHP 7