Remove auto-forward for single-verifier agencies

[thekaveman]

In #337 we introduced handling for an eligibility verifier selection form, which supports choosing between e.g. verifying eligibility with Login.gov or with a Courtesy Card. We also added a shortcut for when an agency has only a single verifier -- to bypass this form and go straight to the next step in the flow.

We want to have a consistent user experience regardless of the number of verifiers, and so it's time to remove this shortcut and always show the verifier selection page, even with only 1 option.

Acceptance Criteria

• The user must always choose a verifier on /eligibility, regardless of the number of verifiers there are to choose from

Additional context

Design sign-off on this approach: https://cal-itp.slack.com/archives/C013W8RUAMU/p1673542656882989