Clean up keyring bindings (#2640)

Remove the config file's `keyring` section and the binding in java code.

core/src/main/java/google/registry/config/RegistryConfig.java

@@ -1099,12 +1099,6 @@ public final class RegistryConfig {
       return config.registryPolicy.greetingServerId;
     }
 
-    @Provides
-    @Config("activeKeyring")
-    public static String provideKeyring(RegistryConfigSettings config) {
-      return config.keyring.activeKeyring;
-    }
-
     @Provides
     @Config("customLogicFactoryClass")
     public static String provideCustomLogicFactoryClass(RegistryConfigSettings config) {

core/src/main/java/google/registry/config/RegistryConfigSettings.java

@@ -37,7 +37,6 @@ public class RegistryConfigSettings {
   public Monitoring monitoring;
   public Misc misc;
   public Beam beam;
-  public Keyring keyring;
   public RegistryTool registryTool;
   public SslCertificateValidation sslCertificateValidation;
   public ContactHistory contactHistory;
@@ -214,12 +213,6 @@ public class RegistryConfigSettings {
     public int transientFailureRetries;
   }
 
-  /** Configuration for keyrings (used to store secrets outside of source). */
-  // TODO(b/388835696): remove section after updating config files.
-  public static class Keyring {
-    public String activeKeyring;
-  }
-
   /** Configuration options for the registry tool. */
   public static class RegistryTool {
     public String clientId;

core/src/main/java/google/registry/config/files/default-config.yaml

@@ -488,11 +488,6 @@ beam:
   initialWorkerCount: 24
   stagingBucketUrl: gcs-bucket-with-staged-templates
 
-keyring:
-  # The name of the active keyring, either "Dummy" or "CSM". The latter stands
-  # for Cloud SecretManager.
-  activeKeyring: Dummy
-
 # Configuration options relevant to the "nomulus" registry tool.
 registryTool:
   # OAuth client ID used by the tool.