Serializable mixin should be removed

[corranwebster]

The class is not used by any current code, appears to be broken, and the problem it was meant to solve (selection of which traits to pickle) is better solved via the use of transient traits metadata.

Technically this is a backwards-incompatible change, however.

[rahulporuri]

@corranwebster it's not clear to me what all code can be thrown out but can we throw out the persistence methods e.g. __getstate__ along with _post_init.

[corranwebster]

I was referring to this class: https://github.com/enthought/chaco/blob/e3423614d654a203bd82ee087c085abe13b465b1/chaco/serializable.py and its tests.

[corranwebster]

Also __getstate__ methods should not be thrown out - it is standard Python stuff: https://docs.python.org/3/library/pickle.html#object.__getstate__

We do need to make sure it behaves nicely with Traits __getstate__: https://github.com/enthought/traits/blob/5f2effc0254570dfa7736d8bf53eb224c9877c57/traits/has_traits.py#L1267