Remove Yahoo and Microsoft provider ID. (#3182)

Author: Yue-Wang-Google

Firebase/Auth/Source/AuthProvider/FIRAuthProvider.m

@@ -39,12 +39,6 @@
 // Declared 'extern' in FIRGameCenterAuthProvider.h
 NSString *const FIRGameCenterAuthProviderID = @"gc.apple.com";
 
-// Declared 'extern' in FIROAuthProvider.h
-NSString *const FIRYahooAuthProviderID = @"yahoo.com";
-
-// Declared 'extern' in FIROAuthProvider.h
-NSString *const FIRMicrosoftAuthProviderID = @"hotmail.com";
-
 #pragma mark - sign-in methods constants
 
 // Declared 'extern' in FIRGoogleAuthProvider.h