improve/simplify validations

[tedteng]

It's better now but I still think it's hard to understand and extend. I think these validations started simple but with added functionality started to get more and more complex. I do not have a concrete proposal now but I'm sure this can be expressed simpler. Maybe something to improve/simplify in a different PR

Originally posted by @petersutter in https://github.com/gardener/gardenctl/pull/404#discussion_r514118493

[tedteng]

	if !IsTargeted(targetReader, "shoot") && (args[0] != "gardener-apiserver") && (args[0] != "gardener-controller-manager") && (args[0] != "tf") && (args[0] != "kubernetes-dashboard") {
		return errors.New("No shoot targeted")
	} else if !IsTargeted(targetReader, "seed") && (args[0] == "tf") || !IsTargeted(targetReader, "shoot") && (args[0] == "tf") {
		return errors.New("No seed or shoot targeted")
	} else if !IsTargeted(targetReader) {