How can I remove the title bar on Ubuntu 19.04?

There are two kinds of titlebars, the "classical" title bar provided as a Windows decoration and, for GTK3 applications only, the new "Client side decorations" (CSD), where the title bar is provided by the application itself, and as such can contain other functional elements such as buttons etc.

All solutions you tried remove the "classical" title bar. They will work on 19.04 provided you run xorg, and may not necessarily work on Wayland. So first check whether you are running xorg or wayland. If you are running Wayland, you can switch to xorg by logging out, then before logging back in, you can select a session on xorg after clicking the cog wheel.

If your application uses a CSD titlebar, then it cannot be removed.