Drop support for deprecated way to pass events to reverse proxy endpoints

[kevinbader]

We support two ways of passing an event to a non-HTTP reverse proxy endpoint:

• Passing a CloudEvent and include a target_partition field in the event's rig extension.
• Passing a CloudEvent in an event field and the target partition next to it in a partition field.

Support for the latter should be dropped with the 3.0 release.