Unable to set X default (no theme) mouse cursor

Testing with current Lubuntu x86-64 16.04 alpha.

There doesn't seem to be any way to choose the X default mouse cursors, i.e. to *not* use cursors from a theme.

In previous Lubuntu releases, a workaround was to uninstall the dmz-cursor-theme package. With no installed cursor themes the system defaults to the X built-in one. However with the 16.04 alpha, the Adwaita cursor theme seems to always be installed, with no way to remove it (the Remove button is greyed out).

It would be nice if a [none] entry could be added to the selectable themes, to allow no cursor theme to be used, without needing to remove all cursor theme packages. (Though as mentioned that workaround no longer works in 16.04.)