Short story: this machine is a non-dedicated server running 24/7, serving X sessions to multiple user accounts and I do not want normal users to be able to suspend/reboot/shutdown this machine form the MDM login screen.
Hence I disabled the suspend/reboot/shutdown commands in mdm.conf, adding overriding lines according to the explanations written in /usr/share/mdm/defaults.conf and it is fine (clicking on the icons only reloads the login screen). But I am still with these now useless suspend/reboot/shutdown icons. Where do I need to look in order to remove them from the MDM login screen ? The line "SystemMenu=false" in the [greeter] section (should but) has no effect on the display of the icons.