chore(CLI): Remove unused alpha CLI subcommand

The Linkerd CLI has a `alpha` subcommand that was previously used to hold experimental subcommands such as ones related to SMI.  All experimental subcommands have been removed and therefore the `alpha` subcommand is empty. This causes a pointless section to be printed at the end of the linkerd CLI usage help:

```
[...]
Additional help topics:
  linkerd alpha        experimental subcommands for Linkerd

Use "linkerd [command] --help" for more information about a command.
```

We remove this empty command so that it no longer shows up in the usage help:

```
Use "linkerd [command] --help" for more information about a command.
```

Signed-off-by: Alex Leong <[email protected]>

Author: adleong

cli/cmd/alpha.go

@@ -104,7 +104,6 @@ func init() {
 	RootCmd.PersistentFlags().StringArrayVar(&impersonateGroup, "as-group", []string{}, "Group to impersonate for Kubernetes operations")
 	RootCmd.PersistentFlags().StringVar(&apiAddr, "api-addr", "", "Override kubeconfig and communicate directly with the control plane at host:port (mostly for testing)")
 	RootCmd.PersistentFlags().BoolVar(&verbose, "verbose", false, "Turn on debug logging")
-	RootCmd.AddCommand(newCmdAlpha())
 	RootCmd.AddCommand(newCmdCheck())
 	RootCmd.AddCommand(newCmdCompletion())
 	RootCmd.AddCommand(newCmdDiagnostics())