Revert "Test suite: run Python doctests for the new tails-gdm-error-message script"

This reverts commit c3b98dbc.

These tests will unavoidably fail on Python << 3.6,
due to lack of dict ordering predictability:

  https://docs.python.org/3.8/library/doctest.html#warnings

Unfortunately, our Jenkins isotesters currently run Stretch,
which has only Python 3.5.