Align types in Rust template with D template

assets/parser.d.erb

@@ -322,8 +322,6 @@ private enum size_t INVALID_ID = cast(size_t)-1;
  *
  * @param input
  *   Text input.
- * @param input_length
- *   Text input length.
  *
  * @return Context structure for lexer/parser.
  */