Remove "cluster" as an alias for "service" command in photon CLI.
Remove "cluster" as an alias for "service" command in photon CLI.

Cluster API is being completely removed from the photon APIs.
Service APIs replaced the ClusterAPIs completely and we no longer need the
alias.
After this  "photon cluster" command won't work. Only "photon service" should work.

Change-Id: I8cf03cdadf08f887d3f91b156201abc034942e3d
(cherry picked from commit dc752255df8b316f30dca9744df7362c17d00c74)

vmware/photon-controller-cli,vmware/photon-controller-cli