Remove reference to non-existent protocol. (#9095)

Author: rosalyntan

FirebaseAuth/Sources/Public/FirebaseAuth/FIRAuth.h

@@ -27,7 +27,6 @@
 @class FIRAuthDataResult;
 @class FIRAuthSettings;
 @class FIRUser;
-@protocol FIRAuthStateListener;
 @protocol FIRAuthUIDelegate;
 @protocol FIRFederatedAuthProvider;