FIX Drop already-verified check from SessionStore

[Cheddam]

Description

While ostensibly a useful safety check, this validation step is superfluous. The purpose of SessionStore is to hold context for the verification process, not to verify its integrity, and there exist (extremely hard-to-reproduce) edge-cases where this will unhelpfully break the MFA flow.

The MFA method verification process is sufficiently idempotent and safe to prevent any negative outcomes from a method being verified multiple times in a single MFA flow.

Manual testing steps

See reproduction steps in related issue.

Issues

• Verification interruptions can break MFA flow #589

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