[RLlib] Fix bug in BC config (automatically setting new API stack related settings).

[sven1977]

Fix bug in BC config (automatically setting new API stack related settings).

The BCConfig c'tor would automatically set RLModule+Learner to True, if EnvRunner+Connector is True.

The desired behavior is to issue an error.
Also, it's not desired to have the any RLlib algo-config class magically set certain settings w/o warning the user or throwing an error.

Why are these changes needed?

Related issue number

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    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
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• Testing Strategy
  • Unit tests
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  • This PR is not tested :(

[simonsays1980]

LGTM.