Migrate from interop pods to source_files header access (#5760)

FirebaseAuthInterop.podspec

@@ -1,31 +0,0 @@
-Pod::Spec.new do |s|
-  s.name             = 'FirebaseAuthInterop'
-  s.version          = '1.1.0'
-  s.summary          = 'Interfaces that allow other Firebase SDKs to use Auth functionality.'
-
-  s.description      = <<-DESC
-  Not for public use.
-
-  A set of protocols that other Firebase SDKs can use to interoperate with FirebaseAuth in a safe
-  and reliable manner.
-                       DESC
-
-  s.homepage         = 'https://firebase.google.com'
-  s.license          = { :type => 'Apache', :file => 'LICENSE' }
-  s.authors          = 'Google, Inc.'
-
-  # NOTE that these should not be used externally, this is for Firebase pods to depend on each
-  # other.
-  s.source           = {
-    :git => 'https://github.com/firebase/firebase-ios-sdk.git',
-    :tag => 'AuthInterop-' + s.version.to_s
-  }
-  s.social_media_url = 'https://twitter.com/Firebase'
-  s.ios.deployment_target = '8.0'
-  s.osx.deployment_target = '10.11'
-  s.tvos.deployment_target = '10.0'
-  s.watchos.deployment_target = '6.0'
-
-  s.source_files = 'Interop/Auth/**/*.h'
-  s.public_header_files = 'Interop/Auth/Public/*.h'
-end