Rip out weak mode #2213

There's currently something called "weak mode" that's labeled experimental, is used by nobody (AFAICT), likely buggy, and confuses readers of the implementation. Nobody cares about owning it. Therefore I propose to rip this out.

This would also close:

• Weak type checking mode Weak type checking mode #1038
• Rethink detection of weakmode Rethink detection of weakmode #925

In its place, we can now specify various flags like check_untyped_defs or disallow_untyped_defs in glob-sections of mypy.ini (see http://mypy.readthedocs.io/en/latest/config_file.html#per-file-flags)