Remove deprecated API in Firebase Auth.

[Yue-Wang-Google]

we should remove deprecated APIs such as

• FIRAuthUpdatedCredentialKey and FIRAuthErrorNameKey in FIRAuthErrors.h
• FIREmailPasswordAuthProviderID and FIREmailPasswordAuthProvider in FIREmailAuthProvider.h

in the next major release.

[ryanwilson]

@Yue-Wang-Google please make sure this is completed by EOD Tuesday (before we cut the release branch).

[Yue-Wang-Google]

#2723 already removed all of those in public interface. I'm going to send another PR to clean up tests and implementation which has those fields.

[Yue-Wang-Google]

#2759 submitted.

closing this bug

[ryanwilson]

Thank you!