Remove deprecated code

[ob-stripe]

r? @brandur-stripe
cc @stripe/api-libraries

Cleanup for version 2.0:

• remove bin/stripe client
• remove a lot of deprecated methods in APIRequestor
• remove deprecated all() method for list requests (the proper method is list())
• remove legacy subscription management methods in Customer
• remove support for api_version argument in EphemeralKey.create() (the proper argument is stripe_version)
• remove delete() method in Source (the proper method is detach())
• remove the comment about removing arg in delete_discount() methods in Customer and Subscription. Instead, support sending params to the API. (Those methods currently don't take accept any parameters, but generally the library doesn't try to enforce which methods support parameters and which don't. Invalid arguments will be declined server-side.)
• remove warning on StripeObject.to_dict(). The method is trivial, might as well keep supporting it.
• clean up global stripe namespace. Invididual exception classes are no longer imported directly in the global namespaces, as well as other internal classes and methods.

[brandur-stripe]

Geeze, I didn't even know this was a thing.

[brandur-stripe]

Amazing that you've been keeping track of all these little inconsistencies all along. Nice!

LGTM.

[ob-stripe]

Amazing that you've been keeping track of all these little inconsistencies all along.

Haha, tbh I mostly grep'd for "deprec", "warnings", "TODO", etc. :)