cli/command/stack/swarm: waitOnServices remove redundant check for multi-error

This check was redundant, because `errors.Join` already checks if the
list of errors is either empty, or only contains `nil` errors, as can
be seen in [this example][1];

    package main

    import (
        "errors"
        "testing"
    )

    func TestMultiErr(t *testing.T) {
        var errs []error
        if err := errors.Join(errs...); err != nil {
            t.Fatal(err)
        }

        errs = append(errs, nil, nil, nil)
        t.Logf("errs contains %d elements", len(errs))
        if err := errors.Join(errs...); err != nil {
            t.Fatal(err)
        }

        errs = append(errs, errors.New("with an error"))
        if err := errors.Join(errs...); err == nil {
            t.Fatal("expected an error")
        }
    }

[1]: https://go.dev/play/p/iSuGP81eght

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

cli/command/stack/swarm/deploy_composefile.go

@@ -292,10 +292,5 @@ func waitOnServices(ctx context.Context, dockerCli command.Cli, serviceIDs []str
 			errs = append(errs, fmt.Errorf("%s: %w", serviceID, err))
 		}
 	}
-
-	if len(errs) > 0 {
-		return errors.Join(errs...)
-	}
-
-	return nil
+	return errors.Join(errs...)
 }