[clock] Use the polkit helper from g-s-d to set time/timezone

gnome-clock-applet-mechanism has moved to gnome-settings-daemon (since
2.31.4) and is now named gsd-datetime-mechanism. The D-Bus interface is
the same, except for the name of the service and the name of the
interface. So we simply need to remove a lot of code that is now
duplicated.

Note that this means polkit is not needed anymore to build gnome-panel.