Delay during unregistering metrics is unnecessary

Discovered while debugging YARN-9071

Quoting billie.rinaldi

I looked at YARN-3619, where the unregistration delay was added. It seems like this was added because unregistration was performed in getMetrics, which was causing a ConcurrentModificationException. However, unregistration was moved from getMetrics into the finished method (in the same patch), and this leads me to believe that the delay is never needed. I'm inclined to think we should remove the delay entirely, but would like to hear other opinions.