This commits removes superfluous checks when using the isc_refcount API.

Examples of superfluous checks:

1. The isc_refcount_decrement function ensures there was not underflow,
   so this check is superfluous:

    INSIST(isc_refcount_decrement(&r) > 0);

2 .The isc_refcount_destroy() includes check whether the counter
   is zero, therefore this is superfluous:

    INSIST(isc_refcount_decrement(&r) == 1 && isc_refcount_destroy(&r));