Remove Google-internal auth mechanism

Nomulus tool will be using OAuth to communicate to GAE endpoints exclusively starting with next week's release. As far as I can tell that is the only part of the system that uses Google-internal auth). We can therefore remove the it after next week's release.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=225221102

java/google/registry/request/auth/AuthModule.java

@@ -30,8 +30,7 @@ public class AuthModule {
   @Provides
   ImmutableList<AuthenticationMechanism> provideApiAuthenticationMechanisms(
       OAuthAuthenticationMechanism oauthAuthenticationMechanism) {
-    return ImmutableList.of(
-        oauthAuthenticationMechanism);
+    return ImmutableList.of(oauthAuthenticationMechanism);
   }
 
   /** Provides the OAuthService instance. */