Make dns_catz_get_iterator() return void

Make dns_catz_get_iterator() return void

Previously, the function(s) in the commit subject could fail for various
reasons - mostly allocation failures, or other functions returning
different return code than ISC_R_SUCCESS.  Now, the aforementioned
function(s) cannot ever fail and they would always return ISC_R_SUCCESS.

Change the function(s) to return void and remove the extra checks in
the code that uses them.