Ubuntu – How to remove top panel on dual screen on secondary monitor

I think its simpler than that. Just set "show on" to "All displays" in the Dock settings on Ubuntu 17.10:

CLI method:

You can disable the multiple monitors option for the dock by running the following command in Terminal:

gsettings set org.gnome.shell.extensions.dash-to-dock multi-monitor false

Then you can set your preferred monitor by running

gsettings set org.gnome.shell.extensions.dash-to-dock preferred-monitor <monitor-number>

(Put -1 in place of <monitor-number> for the primary monitor)

GUI method:

See the answer by singrium and the answer by Dan Dascalescu.
_{(GUI method may not work in special cases)}