commands: simplify passing stdin to the build when the monitor is configured

The monitor needs stdin to run and isn't compatible with loading a
context or dockerfile from stdin. We already disallow this combination
and, with the removal of the remote controller, there's no way to use
stdin during the build when invoke is configured.

This just removes the extra code to allow forwarding stdin to the build
when the monitor is configured to simplify that section of code.

Signed-off-by: Jonathan A. Sternberg <jonathan.sternberg@docker.com>

monitor/monitor.go

@@ -346,10 +346,6 @@ func (m *monitor) invoke(ctx context.Context, pid string, cfg *controllerapi.Inv
 
 func (m *monitor) Close() error {
 	m.cancelRunningProcesses()
-	// if m.buildConfig.resultCtx != nil {
-	// 	b.buildConfig.resultCtx.Done()
-	// }
-	// TODO: cancel ongoing builds?
 	return nil
 }