Cleanup of inferences in arithmetic theory (#5927)

Cleanup of inferences in arithmetic theory (#5927)

This PR cleans up several issues in the arithmetic theory mostly related to its usage of InferenceId and the TheoryInferenceManager:

    remove the ArithLemma class and uses SimpleTheoryLemma instead
    only use NlLemma if we actually need it
    use proper InferenceIds everywhere
    remove unused code in the nonlinear extension