[FirebaseCore] Update headers in FirebaseCore.podspec (#9661)

* [skip ci] Update `FirebaseCore.podspec` headers

* Kick off CI

* [skip ci] Revert "Kick off CI"

This reverts commit a8782b0.

Author: ncooke3

FirebaseCore.podspec

@@ -39,11 +39,7 @@ Firebase Core includes FIRApp and FIROptions which provide central configuration
 
   s.swift_version = '5.3'
 
-  s.public_header_files = [
-    'FirebaseCore/Sources/Public/FirebaseCore/*.h',
-    'FirebaseCore/Extension/*.h',
-  ]
-  s.private_header_files = 'FirebaseCore/Extension/*.h'
+  s.public_header_files = 'FirebaseCore/Sources/Public/FirebaseCore/*.h'
 
   s.framework = 'Foundation'
   s.ios.framework = 'UIKit'