[2.0 spring cleaning] Deprecate subdags

[James Meickle]

I think we should deprecate SubDAGs given the complexity they add and the
limited usage and use cases. Or, we should invest effort in redesigning
their API and implementation. I think that having to account for
subdag-introduced complexity makes Airflow's code much harder to maintain
and buggier, looking at how many open issues there are that reference
subdags (and how unrelated in topic they are):
https://issues.apache.org/jira/browse/AIRFLOW-3292?jql=project%20%3D%20AIRFLOW%20AND%20status%20%3D%20Open%20AND%20text%20~%20%22subdag%22