Remove unused 'using' declarations (#8919)

Author: dconeybe

Firestore/Example/Tests/Util/FSTIntegrationTestCase.mm

@@ -55,7 +55,6 @@
 namespace util = firebase::firestore::util;
 
 using firebase::firestore::core::DatabaseInfo;
-using firebase::firestore::credentials::AuthToken;
 using firebase::firestore::credentials::CredentialChangeListener;
 using firebase::firestore::credentials::EmptyAuthCredentialsProvider;
 using firebase::firestore::credentials::EmptyAppCheckCredentialsProvider;

Firestore/Source/API/FSTFirestoreComponent.mm

@@ -38,7 +38,6 @@
 #include "Firestore/core/src/util/hard_assert.h"
 #include "absl/memory/memory.h"
 
-using firebase::firestore::credentials::CredentialsProvider;
 using firebase::firestore::credentials::FirebaseAppCheckCredentialsProvider;
 using firebase::firestore::credentials::FirebaseAuthCredentialsProvider;
 using firebase::firestore::remote::FirebaseMetadataProviderApple;