refactor(event processor): Remove plugin interfaces from event processor (#322)

Summary:

This PR removes transformers, interceptors, and callbacks from the event processor. These extensibility features are currently unused in optimizely-sdk.

Test plan:

Updated unit tests. Deleted tests for removed functionality. Manually tested. All other existing unit tests should pass.

Issues:

https://optimizely.atlassian.net/browse/OASIS-5151

Author: mjc1283