refactor(firebase_core, web): remove variant fallback in registerVersion (#17874)

* refactor(firebase_core, web): stop passing variant to registerVersion

* refactor(firebase_core, web): add optional variant parameter to registerVersion

Author: SelaseKay

packages/firebase_core/firebase_core_web/lib/src/interop/core.dart

@@ -66,7 +66,7 @@ void registerVersion(String libraryKeyOrName, String version,
   firebase_interop.registerVersion(
     libraryKeyOrName.toJS,
     version.toJS,
-    variant?.toJS ?? 'flutter-firebase'.toJS,
+    variant?.toJS,
   );
 }