Use default completion command provided by cobra. (#522)

* Use default `completion` command provided by cobra.

`cobra` provides default `completion` subcommand.
It supports bash, zsh, fish, and powershell by default.

* update docs

README.md

@@ -480,11 +480,11 @@ eStargz-optimized images.
 
 ## Does `ko` support autocompletion?
 
-Yes! `ko completion` generates a Bash completion script, which you can add to
-your `bash_completion` directory:
+Yes! `ko completion` generates a Bash/Zsh/Fish/PowerShell completion script.
+You can get how to load it from help document.
 
 ```
-ko completion > /usr/local/etc/bash_completion.d/ko
+ko completion [bash|zsh|fish|powershell] --help
 ```
 
 Or, you can source it directly:

doc/ko.md

@@ -16,7 +16,6 @@ ko [flags]
 
 * [ko apply](ko_apply.md)	 - Apply the input files with image references resolved to built/pushed image digests.
 * [ko build](ko_build.md)	 - Build and publish container images from the given importpaths.
-* [ko completion](ko_completion.md)	 - Output shell completion code (default Bash)
 * [ko create](ko_create.md)	 - Create the input files with image references resolved to built/pushed image digests.
 * [ko delete](ko_delete.md)	 - See "kubectl help delete" for detailed usage.
 * [ko deps](ko_deps.md)	 - Print Go module dependency information about the ko-built binary in the image

pkg/commands/commands.go

@@ -32,7 +32,6 @@ func AddKubeCommands(topLevel *cobra.Command) {
 	addBuild(topLevel)
 	addRun(topLevel)
 	addDeps(topLevel)
-	addCompletion(topLevel)
 }
 
 // check if kubectl is installed