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]

After #2430 is done and merged, we'll need to change the base on this to master

[monfresh]

This method doesn't seem to be used anywhere. Can we remove it?

[monfresh]

LGTM % comment

[jmhooper]

I tried rebasing this, but it's so far behind that it is hardly useful anymore. I'm going to close an open another after more cleanup.