How do I remove top bar on secondary monitor??
This is currently not supported. I think it's supposed to be an feature due to the global menu thing.
You can use Gnome extension for this: Gnome extension - Hide Top Bar
This is a problem in professional use-case like using on corporate-events for presentations or video-playback - if you only have one notebook on the projector and no seamless-switcher. On Windows and Mac you can hide the panel on the second screen and i.e. use black or a company-logo as desktop-background. so if you are working on the presentation, there is a black screen or the company logo showing. It is much neater then having a panel on top of the screen...
You can use this Multi-Monitor Gnome Extension. Turn "Show Panel on additional monitors" to OFF in the settings.