Remove ISCAPI_TASK_VALID checks
    
    Recent BIND 9.16.16 has hidden task structure and public checks. It
    still checks task validity in each isc_task_* function, which we use for
    working with tasks. No lesser checking would be done.
    
    isc_mem_t still has them, but they were removed in 9.17. Remove those
    checks as well, work with them just structure undefined pointers. Rely
    on bind9 library to check them.