test [nfc]: Cut redundant checks-extensions from compose_box_checks

These types live in lib/model/autocomplete.dart, so the corresponding
checks-extensions live in test/model/autocomplete_checks.dart (which
indeed has them).

Author: gnprice

test/widgets/compose_box_checks.dart

@@ -1,16 +1,6 @@
 import 'package:checks/checks.dart';
-import 'package:zulip/model/autocomplete.dart';
 import 'package:zulip/widgets/compose_box.dart';
 
 extension ComposeContentControllerChecks on Subject<ComposeContentController> {
   Subject<List<ContentValidationError>> get validationErrors => has((c) => c.validationErrors, 'validationErrors');
 }
-
-extension AutocompleteIntentChecks on Subject<AutocompleteIntent> {
-  Subject<int> get syntaxStart => has((i) => i.syntaxStart, 'syntaxStart');
-  Subject<AutocompleteQuery> get query => has((i) => i.query, 'query');
-}
-
-extension UserMentionAutocompleteResultChecks on Subject<UserMentionAutocompleteResult> {
-  Subject<int> get userId => has((r) => r.userId, 'userId');
-}