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

Commit 9531d26 removed all of ":avocado: enable" tags, but then
a new entry was added with the introduction of migration.py.

Let's remove it for consistency.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
Reviewed-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190607152223.9467-4-crosa@redhat.com>
Signed-off-by: Cleber Rosa <crosa@redhat.com>

tests/acceptance/migration.py

@@ -17,9 +17,6 @@
 
 
 class Migration(Test):
-    """
-    :avocado: enable
-    """
 
     timeout = 10