manifest: Remove plymouth
    
    This dates to https://pagure.io/fedora-atomic/c/36d18b490529fec91b74ca9b464adb73ef0ab462
    
    As far as why plymouth is in `@core` as mandatory...I don't know.
    It certainly doesn't seem like it should be mandatory.  OK, some
    investigation shows it appeared in `comps-f17.xml.in`, and annotate
    links to https://pagure.io/fedora-comps/c/ed64ea886543e82b74ed7a26ac9ad5ab31591ef6
    
    Hm, which raises the question of whether or not our support for LUKS
    without plymouth works.  But honestly...for servers you really
    want https://github.com/latchset/tang