command: Remove the experimental "terraform add" command

We introduced this experiment to gather feedback, and the feedback we saw
led to us deciding to do another round of design work before we move
forward with something to meet this use-case.

In addition to being experimental, this has only been included in alpha
releases so far, and so on both counts it is not protected by the
Terraform v1.0 Compatibility Promises.

commands.go

@@ -109,12 +109,6 @@ func initCommands(
 	// that to match.
 
 	Commands = map[string]cli.CommandFactory{
-		"add": func() (cli.Command, error) {
-			return &command.AddCommand{
-				Meta: meta,
-			}, nil
-		},
-
 		"apply": func() (cli.Command, error) {
 			return &command.ApplyCommand{
 				Meta: meta,