Do not create robots.txt by default

Meny people, including me, have a custom robots.txt in static.

Also remove that option from the command line; it doesn't feel
important enough.

Fixes ##2049

hugolib/site.go

@@ -1864,7 +1864,7 @@ func (s *Site) RenderSitemap() error {
 }
 
 func (s *Site) RenderRobotsTXT() error {
-	if viper.GetBool("DisableRobotsTXT") {
+	if !viper.GetBool("EnableRobotsTXT") {
 		return nil
 	}