Break SessionSource out of SessionMetadata and rename it EppRequestSource.

The "SessionSource" has nothing to do with sessions (and it's often
used in sessionless contexts). What it does indicate is the endpoint
used to make the request.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=125295224

java/google/registry/flows/StatelessRequestSessionMetadata.java

@@ -21,15 +21,12 @@ public class StatelessRequestSessionMetadata extends SessionMetadata {
 
   private final String clientId;
   private final Set<String> serviceExtensionUris;
-  private final SessionSource sessionSource;
 
   public StatelessRequestSessionMetadata(
       String clientId,
-      Set<String> serviceExtensionUris,
-      SessionSource source) {
+      Set<String> serviceExtensionUris) {
     this.clientId = clientId;
     this.serviceExtensionUris = serviceExtensionUris;
-    this.sessionSource = source;
   }
 
   @Override
@@ -42,11 +39,6 @@ public class StatelessRequestSessionMetadata extends SessionMetadata {
     return serviceExtensionUris;
   }
 
-  @Override
-  public SessionSource getSessionSource() {
-    return sessionSource;
-  }
-
   @Override
   public void invalidate() {
     throw new UnsupportedOperationException();