removed unnecessary `asynchronous` decorator.

**Summary:**
- removed unnecessary `asynchronous` decorator.

According to asynchronous method documentation, `asynchronous` is unnecessary if used with ``@gen.coroutine``:
```python
def asynchronous(method):
    """Wrap request handler methods with this if they are asynchronous.

    This decorator is unnecessary if the method is also decorated with
    ``@gen.coroutine`` (it is legal but unnecessary to use the two
    decorators together, in which case ``@asynchronous`` must be
    first).
```