cmd: remove custom completion subcommand

Since the release of cobra v1.2.0, a completion command is automatically
created if none is provided. We therefore do not need to ship our own
anymore.

Note that this means two breaking changes:

- Calling `cilium completion` without specifying a shell name no leads
  to printing out the help message for the command. Our implementation
  used to generate completion for bash. I think that, although breaking,
  this change is for the better.
- We used to support `ps1` as an alias for `powershell` and the builtin
  subcommand does not.

What we get is more uniformity for our users. Most CLI tools using cobra
are likely to use the built-in command to generate shell completion
rather than rolling their own.

Signed-off-by: Robin Hahling <[email protected]>

internal/cli/cmd/cmd.go

@@ -77,7 +77,6 @@ cilium connectivity test`,
 	cmd.AddCommand(
 		newCmdClusterMesh(),
 		newCmdConfig(),
-		newCmdCompletion(),
 		newCmdConnectivity(),
 		newCmdContext(),
 		newCmdHubble(),