Update system prune flag --all confirmation prompt

[suyanhanx]

As both image.Prune and builder.Prune now support only removing dangling items or not, the confirmation prompt for the flag --all of system prune need to update.

---

I'm sorry that some code changes were reverted due to my mistake in operating the git rebase.

nerdctl/cmd/nerdctl/system_prune.go

Lines 83 to 87 in 17b6166

	if !options.All {
	logrus.Warn("Currently, `nerdctl system prune` requires --all to be specified. Skip pruning.")
	// NOP
	return false, nil
	}

This PR removed them.

Signed-off-by: Han Xu suyanhanx@gmail.com

[AkihiroSuda]

Thanks