Remove Plymouth from Server install_default_upload (#1933378)
    
    This is a workaround for
    https://bugzilla.redhat.com/show_bug.cgi?id=1933378 . Sometimes
    when booting Server on a tap test without a working network
    config (no DHCP server, static network not yet configured) we
    hit a bug where the splash screen does not clear completely, and
    this causes all the console needle matches to fail. To work
    around this, we remove plymouth from the installed system after
    running the install_default_upload test on Server; all affected
    tests use the image uploaded by that test. We exclude aarch64
    because there's a known problem with removing plymouth on that
    arch (#1940163), plus the bug doesn't actually seem to happen on
    aarch64 for some reason.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>