Prune IdV branches from MFA otp verification logic

[jmhooper]

Why: We started using separate otp verification controllers for MFA
and IdV in #2430. This commit is a follow-on to that to remove the code
that supported IdV in the MFA controller.

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[jmhooper]

This PR is meant to replace #2437 which I closed

[jgsmith-usds]

This looks pretty good to me. I'm digging back into the phone management code to start building out the real multi-phone support stuff and it's pretty hairy. This simplifies it a bit. I like simplification.