Remove logic for parsing and diagnosing `jvp:` and `vjp:` arguments for
`@differentiable` attribute. No logic remains for handling those arguments.
Follow-up to TF-1001.
Delete `@differentiable` attribute `jvp:` and `vjp:` arguments for derivative
registration. `@derivative` attribute is now the canonical way to register
derivatives.
Resolves TF-1001.
