Consider deleting RequestsPerSecondInputVariableSetterConfig

I originally created RequestsPerSecondInputVariableSetterConfig to avoid adding extra code to support the config proto being optional. But it turns out that each plugin will be created by copy-paste, so it can choose whether to use a config proto.

On one hand:
- If we always create a proto for every plugin, even if it has no field, it's simpler to understand, and the proto file serves as documentation.
- Theoretically we could think of config fields to add to this proto.
- I don't understand the past or future of the Envoy plugin registry -- for example, what if Envoy starts requiring the value returned from createEmptyConfigProto to uniquely identify the plugin?

On the other hand, it does seem that today we could just ignore the Any and use a zero-arg constructor. If the Envoy registry behavior changes, we can try to add the proto back.

See source/adaptive_load/input_variable_setter_impl.cc.