Remove dead init_deferred_fields method

[joecox]

Problem

The init_deferred_fields method, added in 6ffae1a, had its only usage removed in e6c7b56.

Solution

Remove the init_deferred_fields method, along with the now-unused DescriptorMixin.

Commandments

• Write PEP8 compliant code.
• Cover it with tests.
• Update CHANGES.rst file to describe the changes, and quote according issue with GH-<issue_number>.
• Pay attention to backward compatibility, or if it breaks it, explain why.
• Update documentation (if relevant).

[joecox]

Given that this is removal of dead code, which doesn't affect any documented feature, I opted not to update CHANGES.rst. I can do that if needed.

[foarsitter]

Thanks @joecox