Fix the Dockerfile at infrastructure/docker/traffic_ops/Dockerfile (#4667)
Fix the Dockerfile at infrastructure/docker/traffic_ops/Dockerfile (#4667)

as it references a shell script, traffic_ops/install/bin/download_web_deps
that no-longer exists.  The script was removed with commit
c6a6ca8aed469d9cc6f399e3073d16d9ce5c9178
hbeatty/incubator-trafficcontrol,hbeatty/incubator-trafficcontrol,hbeatty/incubator-trafficcontrol,hbeatty/incubator-trafficcontrol,hbeatty/incubator-trafficcontrol,hbeatty/incubator-trafficcontrol,hbeatty/incubator-trafficcontrol,hbeatty/incubator-trafficcontrol,hbeatty/incubator-trafficcontrol,hbeatty/incubator-trafficcontrol