Revert ios build workaround for Firebase packages

[rajveermalviya]

This reverts commit a9b2016.

[PIG208]

Thanks! Looks like I missed this in my review list. I wasn't able to test this manually; from reading the changes and the linked context, this LGTM. Marking this for Greg's review.

[gnprice]

Thanks for cleaning this up!

Let's keep around the Zulip.xcconfig file and the bits of config that wire it up — it'll just become empty except for the general comment at top.

That way the next time there's some Xcode config we want to add, we have all the infrastructure in place to just drop it in that file and have it get applied. Effectively we'll get to re-use the work that we did in #958 to figure those details out.

[rajveermalviya]

Thanks for the review @gnprice! Pushed a new revision, PTAL.

[gnprice]

Thanks! Looks good; merging, with a few commit-message tweaks:

-    Revert "ios: Add workaround for package:firebase_messaging when building with Xcode 16"
+    ios build: Revert workaround for package:firebase_messaging
 
-    This reverts part of commit a9b20167ccf13ebaf6bcd6a28060ca4438f8c8a8,
-    keeping the `Zulip.xcconfig` file and it's references.
+    This reverts part of commit a9b20167c (#958),
+    keeping the `Zulip.xcconfig` file and its references.

(In particular, 'Revert "ios: …"' is no longer accurate because it's not reverting that whole commit.)