Remove u2f implementation

For a while now WebAuthn has replaced u2f.
And since web-vault v2.27.0 the connector files for u2f have been removed.
Also, on the official bitwarden server the endpoint to `/two-factor/get-u2f` results in a 404.

- Removed all u2f code except the migration code from u2f to WebAuthn

src/db/models/two_factor.rs

@@ -157,7 +157,7 @@ impl TwoFactor {
                 .from_db()
         }};
 
-        use crate::api::core::two_factor::u2f::U2FRegistration;
+        use crate::api::core::two_factor::webauthn::U2FRegistration;
         use crate::api::core::two_factor::webauthn::{get_webauthn_registrations, WebauthnRegistration};
         use webauthn_rs::proto::*;