ios build: Revert workaround for package:firebase_messaging

This reverts part of commit a9b2016 (zulip#958),
keeping the `Zulip.xcconfig` file and its references.

The fix was released in version `3.6.0` of `package:firebase_core`
and version `15.1.3` of `package:firebase_messaging`:
  firebase/flutterfire@d7d2d4b

We have since then updated Firebase dependencies 3 times:
  zulip@24215f6
  zulip@1abb0a0
  zulip@3ba9985

Author: rajveermalviya

ios/Flutter/Zulip.xcconfig

@@ -1,10 +1,2 @@
 // Configuration settings file format documentation can be found at:
 // https://help.apple.com/xcode/#/dev745c5c974
-
-// TODO(firebase/flutterfire#13323): remove this flag
-//
-// This flag is added to work around the iOS build failing
-// on Xcode 16.  Remove it when `package:firebase_messaging`
-// is patched with a fix to build successfully on Xcode 16:
-//   https://github.com/firebase/flutterfire/issues/13323
-CLANG_ALLOW_NON_MODULAR_INCLUDES_IN_FRAMEWORK_MODULES=YES