Remove unused import (#565)

This PR removes an import of from the `dateutil` package since it is not
being used. The import had been added recently:
5ec257e.

From what I gather, this was the only place that `dateutil` was being
used. Since `dateutil` was not in our `requirements.txt`, this could be
related to the current issue where our readthedocs site is not
displaying the docstring details. Hopefully this fixes the Read the Docs
rendering issue. The build logs show that only section outlines were
rendered because autodoc failed to import `wfdb` after hitting a
now-removed, unused `dateutil` import that wasn’t installed in the docs
build environment.

Author: briangow

wfdb/io/_header.py

@@ -1,5 +1,4 @@
 import datetime
-from dateutil import parser
 from typing import Any, Dict, List, Optional, Sequence, Tuple
 
 import numpy as np