update or remove `__all__` declarations

[jvanasco]

Many of the files in WebOb declare __all__ for exports. This can create problems with the current generation of type checkers, as they may not be able to see functions that do not appear in these declarations. For example, in webob.cookies, parse_cookie is not declared.

The __all__ declarations are only used for * imports - which are increasingly unused in exchange for explicit imports.

I would be happy to generate a PR for either situation, however I would prefer dropping __all__.