simplify: dedupe boilerplate and remove dead code across all layers

lib/api/marianumcloud/autocomplete/autocomplete_api.dart

@@ -1,4 +1,3 @@
-import 'dart:convert';
 import 'dart:io';
 
 import 'package:http/http.dart' as http;
@@ -38,9 +37,6 @@ class AutocompleteApi {
         technicalDetails: 'core/autocomplete/get: ${response.body}',
       );
     }
-    final decoded = jsonDecode(response.body) as Map<String, dynamic>;
-    return AutocompleteResponse.fromJson(
-      decoded['ocs'] as Map<String, dynamic>,
-    );
+    return AutocompleteResponse.fromJson(NextcloudOcs.decode(response.body));
   }
 }