How do I remove the title bar in Xubuntu?

Recent versions of XFCE have this built-in. Check out Settings Manager > Window Manager Tweaks > Accessibility > Hide title of windows when maximized.

Maximized windows will not have the title bar then, while non-maximized windows will have it. Tested on ArchLinux, xfce version 4.12.0-4.

I had the same issue with the titlebar and I solved it to a good level this way:

• Settings > Settings Manager > Window Manager > Theme tab > Kokodi (it has a high titlebar)
• Settings > Settings Manager > Panel > Row Size (pixels) = 28
• Settings > Settings Manager > Panel > Don't reserve space on borders = Yes, turned on

This way, the titlebar is just hidden below the panel. Benefits of such approach:

• The titlebar remains visible for small windows
• The titlebar is autohidden with Alt+F10 (but the panel remains visible in all cases except hard fullscreen Alt + F11)
• If you have "snap to other windows" enabled in your window manager, you can easily get an almost-fullscreen window, with panel and titlebar still visible. Just start to change window size and it'll do a stop in the point where panel starts (allowing to go further though).

The bad thing is that it feels like a hack. I searched this topic because I wanted to find an xfce setting for this (failed yet).

A newer answer notes that a built-in XFCE setting is now available.

Try the Maximus package? Here's the package description:

A desktop daemon which will automatically maximize and, optionally, un-decorate windows. Has to support for exclusion lists and will work with any EWMH specification compliant window-manager.

Install with:

sudo apt-get install maximus

I believe you'll need to add the Maximus command to your startup programs, as it does not automatically run at boot on its own.