simplify: dedupe boilerplate and remove dead code across all layers

lib/model/endpoint_data.dart

@@ -29,13 +29,10 @@ class EndpointData {
 
   EndpointData._construct();
 
-  EndpointMode getEndpointMode() {
-    late String existingName;
-    existingName = AccountData().getUsername();
-    return existingName.startsWith('google')
-        ? EndpointMode.stage
-        : EndpointMode.live;
-  }
+  EndpointMode getEndpointMode() =>
+      AccountData().getUsername().startsWith('google')
+      ? EndpointMode.stage
+      : EndpointMode.live;
 
   Endpoint nextcloud() => EndpointOptions(
     live: Endpoint(domain: 'cloud.marianum-fulda.de'),