Remove un-implemented Scheduler schema

[grayside]

Problem

The proto schema for Cloud Scheduler is not associated with an Eventarc Scheduler integration. It's unclear what it's meant to relate to. As of now it is unhelpful and unusable.

Proposal

Remove this proto and allow code generation processes to remove scheduler from all derived schemas and libraries.

Per separate discussion, this may require some additional thought in managing upstream data and the C# library. cc @jskeet