Get rid of GNOME Flashback, Metacity and friends

At least these packages could probably be removed from the ISO, thanks to the new Greeter:

gnome-flashback-common gnome-menus gnome-panel gnome-panel-data
libpanel-applet0 gnome-flashback gnome-session-flashback
metacity metacity-common

Feature Branch: feature/8230-greeter-revamp-phase1

Original created by @intrigeri on 12064 (Redmine)

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