message: Remove ParametersParser::this
  Summary: Dead code.
diff: @@ -683,11 +683,6 @@ impl<'v, 'a> ParametersParser<'v, 'a> {
  v.get()
  }
  - pub fn this<T: UnpackValue<'v>>(&self, this: Option<Value<'v>>) -> anyhow::Result<T> {
  - let this = this.ok_or(ValueError::MissingThis)?;
  - T::unpack_named_param(this, "this")
  - }
  -
  /// Obtain the next parameter, corresponding to [`ParametersSpec::optional`].
  /// It is an error to request more parameters than were specified.
  /// The `name` is only used for error messages.
  