node(cleanup): remove unused supervisor code (wormhole-foundation#4388)

- Removes test helpers for the super visor. The supervisor's tests are
  based on the upstream repo (documented in UPSTREAM.md). They were
  removed in 585a22c. As a result,
  these helpers functions are unused.
- Remove `Command` function as it is not used in the codebase.

Author: johnsaigle

node/pkg/supervisor/supervisor_support.go

@@ -5,7 +5,6 @@ package supervisor
 import (
 	"context"
 	"net"
-	"os/exec"
 
 	"google.golang.org/grpc"
 )
@@ -34,13 +33,3 @@ func GRPCServer(srv *grpc.Server, lis net.Listener, graceful bool) Runnable {
 		}
 	}
 }
-
-// Command will create a Runnable that starts a long-running command, whose exit is determined to be a failure.
-func Command(name string, arg ...string) Runnable {
-	return func(ctx context.Context) error {
-		Signal(ctx, SignalHealthy)
-
-		cmd := exec.CommandContext(ctx, name, arg...)
-		return cmd.Run()
-	}
-}