DOC: Remove unused/flaky statsmodels intersphinx_mapping

[mroeschke]

The doc build can occasionally fail with

2022-06-21T19:46:59.4564919Z WARNING: failed to reach any of the inventories with the following issues:
2022-06-21T19:46:59.4567149Z intersphinx inventory 'https://www.statsmodels.org/devel/objects.inv' not fetchable due to <class 'requests.exceptions.ConnectionError'>: HTTPSConnectionPool(host='www.statsmodels.org', port=443): Max retries exceeded with url: /devel/objects.inv (Caused by NewConnectionError('<urllib3.connection.HTTPSConnection object at 0x7fd05f1ee8b0>: Failed to establish a new connection: [Errno -3] Temporary failure in name resolution'))

Looking through our docs, we don't have a sphinx reference to the statsmodel docs so probably safe to remove to avoid flakiness.