Remove outdated comment in firestore.yml (#9560)

Author: ncooke3

.github/workflows/firestore.yml

@@ -273,7 +273,6 @@ jobs:
     - name: Swift Build
       run: scripts/third_party/travis/retry.sh ./scripts/build.sh FirebaseFirestoreSwift-Beta ${{ matrix.target }} spmbuildonly
 
-# Restore when FirebaseUI works with Firebase 7 (#6646)
   quickstart:
     # Don't run on private repo unless it is a PR.
     if: (github.repository == 'Firebase/firebase-ios-sdk' && github.event_name == 'schedule') || github.event_name == 'pull_request'