Remove old assertion in resync replication request (elastic#42390)

This assertion was left behind from a previous cleanup. The assertion
was there to remove some stale logic not needed when master would not
talk to 6.x anymore. When that logic was removed, this assertion was
left behind. This commit removes that stale assertion.

server/src/main/java/org/elasticsearch/action/resync/ResyncReplicationRequest.java

@@ -38,8 +38,6 @@ public final class ResyncReplicationRequest extends ReplicatedWriteRequest<Resyn
     private final long maxSeenAutoIdTimestampOnPrimary;
 
     ResyncReplicationRequest(StreamInput in) throws IOException {
-        //  TODO: https://github.com/elastic/elasticsearch/issues/38556
-        //assert Version.CURRENT.major <= 7;
         super(in);
         trimAboveSeqNo = in.readZLong();
         maxSeenAutoIdTimestampOnPrimary = in.readZLong();