kdumpctl: Remove slub_debug from cmdline
    
    slub_debug parameter enables debug for slub, making each object take
    more memory than normal. During a typical kdump, "slub_debug=FZPU" will
    cost about 33MB additional memory. If users really want to enable this
    parameter, they should specify it in KDUMP_COMMANDLINE_APPEND.
    
    Signed-off-by: Dangyi Liu <dliu@redhat.com>