Make isc_ht_init() and isc_ht_iter_create() return void [v9.18]

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.

(cherry picked from commit 8fa27365)