LG-7483: Remove feature flag select_multiple_mfa_options

[aduth]

🎫 Ticket

LG-7483

🛠 Summary of changes

Removes the select_multiple_mfa_options feature flag, which is now live in all environments.

Also fixes an issue where MFA "Delete" links would not be shown on the account dashboard in some circumstances, since that was intended to apply only for Kantara restrictions (not live in production), but instead had been keying off the select_multiple_mfa_options feature (live in production).

📜 Testing Plan

1. Check that you can configure multiple MFA for account creation
2. Check that you can delete MFA methods from your account when you have multiple active, regardless of whether the MFA method is "restricted"

The second test procedure requires overrides in local development config/application.yml (until LG-7586):

development:
  kantara_2fa_phone_restricted: false

[aduth]

This code is unreachable, since PhoneSelectionPresenter overrides the info method and therefore setup_info would not be called for those methods. The code here is incorrect anyways, since it's showing the Kantara messaging based on select_multiple_mfa_options (should only be shown with kantara_2fa_phone_restricted).

identity-idp/app/presenters/two_factor_authentication/phone_selection_presenter.rb

Lines 15 to 20 in dc20140

	def info
	IdentityConfig.store.kantara_2fa_phone_restricted &&
	MfaContext.new(user).enabled_mfa_methods_count == 0 ?
	t('two_factor_authentication.two_factor_choice_options.phone_info_html') :
	t('two_factor_authentication.two_factor_choice_options.phone_info')
	end

[jmdembe]

This makes sense. LGTM 👍🏾

[aduth]

I'm a little confused at needing to remove some of these SAML-specific handling, though as I understand why they were introduced in #6894 to trigger the post-auth redirect that's typically handled in JavaScript, I feel like it would be unexpected that a SAML auth would be "complete" at this point. In main, the URL here is the same as what's being asserted below (UriService.add_params(@saml_authn_request, locale: :es)), which feels very unexpected. My guess is that based on the timing of the merge of #6894 being after multiple MFA setup was already live in most environments, this was buggy in situations where the config was turned off (as it had been in test environments), and that the changes here are the correct, expected behavior.