Remove learn method demo

[juhoinkinen]

The default demo usage

$ python3 annif_client.py

was crashing also in the learn step:

* Learning on a document
Traceback (most recent call last):
  File "annif_client.py", line 120, in <module>
    req = annif.learn(project_id='dummy-en', documents=documents)
  File "annif_client.py", line 60, in learn
    req.raise_for_status()
  File "/home/juhoi/git/Annif-client/venv/lib/python3.8/site-packages/requests/models.py", line 1021, in raise_for_status
    raise HTTPError(http_error_msg, response=self)
requests.exceptions.HTTPError: 403 Client Error: Forbidden for url: https://api.annif.org/v1/projects/dummy-en/learn

This is because the learn endpoint is denied by NGINX. Better to remove demoing of that method.