Remove unused single country variant from phone input component

[aduth]

🎫 Ticket

Supports LG-14382

🛠 Summary of changes

Updates PhoneInputComponent to remove special consideration for a case where only a single country is supported, which would avoid rendering country list as a dropdown selector.

This was originally added in #5619, specifically for IdV phone verification step. At the time, we only accepted U.S. phone numbers for phone verification. This has since been expanded to support U.S. territories. Therefore, we don't currently use this functionality at all in the application.

These changes facilitate the work being done in #11257, where the single country variant does not work as expected after the upgrade.

Easier to review with whitespace changes hidden: https://github.com/18F/identity-idp/pull/11260/files?w=1

📜 Testing Plan

1. Go to http://localhost:3000/components/inspect/phone_input/workbench
2. Observe no regressions in the behavior of phone input component
3. Go to http://localhost:3000/components/inspect/phone_input/workbench?allowed_countries=US
4. Observe that the phone input component still works even if we only had one option, it just isn't an especially optimized experience

[aduth]

For reference, the default is true:

https://github.com/jackocnr/intl-tel-input/blob/511870c0151d46c9bcbaf0b4b9cdd88a6fbd4096/src/js/intl-tel-input.ts#L85