cloudplugin: Remove incomplete stub of "cloud" subcommand

We seem to have inherited an incomplete implementation of something from
the predecessor project here: a "tofu cloud" command that just tries to
immediately delegate any invocation to another executable called
"terraform-cloudplugin" in the current working directory, used as a
go-plugin style plugin.

This has some TODO comments suggesting that it was intended to change to
download a plugin from some remote place before executing it, but our
stubby version doesn't do that. I was also hidden behind an experimental
feature guard and so has never been accessible in any released version of
OpenTofu; we don't currently produce any releases with experimental
features enabled.

Therefore this commit just deletes it so that we don't have this dead code
to potentially worry about. Perhaps one day we'll offer some extension
point for adding extra subcommands through plugins, but if we do that then
we'll presumably design our own mechanism for doing it rather than
extending this dead code that was added for reasons unknown to us.

Signed-off-by: Martin Atkins <mart@degeneration.co.uk>

cmd/tofu/commands.go

@@ -443,14 +443,6 @@ func initCommands(
 		},
 	}
 
-	if meta.AllowExperimentalFeatures {
-		commands["cloud"] = func() (cli.Command, error) {
-			return &command.CloudCommand{
-				Meta: meta,
-			}, nil
-		}
-	}
-
 	primaryCommands = []string{
 		"init",
 		"validate",