Remove the sub-command

[Yuggupta27]

Since the binary will be self-explanatory for execution, we can get rid of the subcommand as it is implicit(the migration will be to CSI and it is already indicated via binary).
We will just accept the parameters to the binary, and not any other command.

Sample usage:
./migrate-to-csi --sourcestoraageclass=rook-ceph-block --destinationstorageclass=csi-rook-ceph-block

cc @travisn