Remove DeviceModel::as_user_facing_str

This patch removes the unused DeviceModel::as_user_facing_str method.
The device model is only used for the argument handling.  When printing
messages for the user, we always use nitrokey::Model.  Its Display
implementation already prints the model in an appropriate format.

src/args.rs

@@ -44,16 +44,6 @@ Enum! {
  ]
 }
 
-impl DeviceModel {
- pub fn as_user_facing_str(&self) -> &str {
- match self {
- DeviceModel::Librem => "Librem",
- DeviceModel::Pro => "Pro",
- DeviceModel::Storage => "Storage",
- }
- }
-}
-
 impl From<DeviceModel> for nitrokey::Model {
  fn from(model: DeviceModel) -> nitrokey::Model {
  match model {