kdumpctl: remove some legacy code
    
    It seems the save_core function and vmcore detection was used a long
    time ago when kdump shares same userspace in first and second kernel.
    It's now heavily deprecated (only support cp, hardcoded path, dumpoops
    no longer exists) and not used.
    
    Now vmcore will never show up in first kernel for both kdump and fadump
    case, and kdumpctl is only used in first kernel, so just remove them.
    
    Signed-off-by: Kairui Song <kasong@tencent.com>
    Acked-by: Coiby Xu <coxu@redhat.com>