Acceptance tests: drop usage of ":avocado: enable"

Acceptance tests: drop usage of ":avocado: enable"

The Avocado test runner attemps to find its INSTRUMENTED (that is,
Python based tests) in a manner that is as safe as possible to the
user.  Different from plain Python unittest, it won't load or
execute test code on an operation such as:

 $ avocado list tests/acceptance/

Before version 68.0, the logic implemented to identify INSTRUMENTED
tests would require either the ":avocado: enable" or ":avocado:
recursive" statement as a flag for tests that would not inherit
directly from "avocado.Test".  This is not necessary anymore,
and because of that the boiler plate statements can now be removed.

Reference: https://avocado-framework.readthedocs.io/en/68.0/release_notes/68_0.html#users-test-writers
Signed-off-by: Cleber Rosa <crosa@redhat.com>
Reviewed-by: Caio Carrara <ccarrara@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Message-Id: <20190218173723.26120-1-crosa@redhat.com>
Signed-off-by: Cleber Rosa <crosa@redhat.com>