kdumpctl: Kill duplicate code related to file modication check
    
    commit "28e8c4b5ac89 kdumpctl: Move file modification check logic in
    check_system_modified()" copied file modification check logic instead of
    moving.
    Kill the duplicate logic from original calling function check_rebuild().
    
    Signed-off-by: Pratyush Anand <panand@redhat.com>
    Reviewed-by: Xunlei Pang <xlpang@redhat.com>
    Acked-by: Dave Young <dyoung@redhat.com>