Allow STYLE_OPTION_MONO to be unset from toolkit-created window

This was keeping form controls (e.g. CheckBox and RadioButton) to
transition from flat style to 3d style, while the 3d-to-flat
transition was allowed, making flat some unescapable pit of doom.

Whatever the goal of that pinning, it will have to be achieved in a
different way.

Change-Id: Icc4dfb9673a7dfe4a7d8726fbfd23f75499f8b41
Reviewed-on: https://gerrit.libreoffice.org/9554
Reviewed-by: Norbert Thiebaud <nthiebaud@gmail.com>
Tested-by: Norbert Thiebaud <nthiebaud@gmail.com>