Doc: remove cluster references in traffic_ctl
Doc: remove cluster references in traffic_ctl

Clustering is gone.

Also removing the metric clear command as it is not implemented.
To achieve that, perhaps delete records.snap.

SolidWallOfCode/trafficserver,chitianhao/trafficserver,SolidWallOfCode/trafficserver,chitianhao/trafficserver,SolidWallOfCode/trafficserver,SolidWallOfCode/trafficserver,vmamidi/trafficserver,pbchou/trafficserver,bryancall/trafficserver,chitianhao/trafficserver,SolidWallOfCode/trafficserver,davidbz/trafficserver,dyrock/trafficserver,vmamidi/trafficserver,duke8253/trafficserver,dyrock/trafficserver,dyrock/trafficserver,davidbz/trafficserver,bryancall/trafficserver,duke8253/trafficserver,bryancall/trafficserver,davidbz/trafficserver,davidbz/trafficserver,davidbz/trafficserver,bryancall/trafficserver,dyrock/trafficserver,vmamidi/trafficserver,duke8253/trafficserver,bryancall/trafficserver,bryancall/trafficserver,davidbz/trafficserver,dyrock/trafficserver,pbchou/trafficserver,vmamidi/trafficserver,dyrock/trafficserver,chitianhao/trafficserver,chitianhao/trafficserver,SolidWallOfCode/trafficserver,duke8253/trafficserver,pbchou/trafficserver,vmamidi/trafficserver,duke8253/trafficserver,chitianhao/trafficserver,duke8253/trafficserver,dyrock/trafficserver,vmamidi/trafficserver,chitianhao/trafficserver,duke8253/trafficserver,pbchou/trafficserver,pbchou/trafficserver,SolidWallOfCode/trafficserver,pbchou/trafficserver