Fix flickering in debug panel

This flickering made debug panel unusable, because most of the time
panel was white. This problem effects wxPython with GTK3+ backend
(at least on GNU/Linux systems).

Removed code IIUC draws box with content of moved debug variable, I
don't see any effect of it neither on GNU/Linux nor on Windows. It
should be safe to remove.

          <xsd:attribute name="Command" type="xsd:string" use="optional" default="make -C %(buildpath)s all BEREMIZSRC=%(src)s BEREMIZCFLAGS=%(cflags)s MD5=%(md5)s USE_BEREMIZ=1 FROM_BEREMIZ=1"/>