Greenfield: Remove unused checkout type setting from POS (#5512)

Cam across this while browsing the API docs: The checkout type setting isn't used for the POS, so we should simply remove it as this is configured on the store-level.

BTCPayServer/Controllers/GreenField/GreenfieldAppsController.cs

@@ -291,8 +291,7 @@ namespace BTCPayServer.Controllers.Greenfield
                 EmbeddedCSS = request.EmbeddedCSS,
                 RedirectAutomatically = request.RedirectAutomatically,
                 RequiresRefundEmail = BoolToRequiresRefundEmail(request.RequiresRefundEmail) ?? RequiresRefundEmail.InheritFromStore,
-                FormId = request.FormId,
-                CheckoutType = request.CheckoutType ?? CheckoutType.V1
+                FormId = request.FormId
             };
         }