Remove outdated comment (#126927)

Follow-up to https://github.com/flutter/flutter/pull/126647#discussion_r1195417860.

packages/flutter/lib/src/foundation/assertions.dart

@@ -1009,7 +1009,7 @@ class FlutterError extends Error with DiagnosticableTreeMixin implements Asserti
       isInDebugMode = true;
       return true;
     }());
-    final bool reportError = isInDebugMode || !details.silent; // could be null
+    final bool reportError = isInDebugMode || !details.silent;
     if (!reportError && !forceReport) {
       return;
     }