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/HttpSessionMetadata.java

@@ -52,9 +52,4 @@ public class HttpSessionMetadata extends SessionMetadata {
   protected Object getProperty(String key) {
     return session.getAttribute(key);
   }
-
-  @Override
-  public SessionSource getSessionSource() {
-    return SessionSource.HTTP;
-  }
 }