Remove requested_vtr_authn_contexts method

[Sgtpluck]

Part 4 of removing VoT from the IdP, following up on 18F/identity-idp#12741

• Removing Vectors of Trust code from the OIDC flow (Remove VoT from OIDC identity-idp#12718).
• Removing VoT code from SAML flow (Remove vot from SAML flow identity-idp#12721)
• Remove references to VoT from the AuthnContextResolver, as well as the references to the Vectors themselves. (Remove vot from resolver identity-idp#12741_)
• Removing VoT from the saml_idp gem (this PR)
• Renaming files so that all references to VoT will be removed, removing related configuration code. (except perhaps some specs that ensure it is not a valid code flow)

[ajfarkas]

pretty straightforward