Expire deprecations and remove dead code

[bonjourmauko]

Job story

When doing the maintenance of openfisca-core,
I expect only the core features to be present in the code base,
So as to make it easier to maintain and to evolve based on the community's needs.

TODO

• A
• long
• list
• of
• thing
• to
• deprecate
• BREAKING CHANGE: cleanup entities #1286