Make isc_heap_create() and isc_heap_insert() return void [v9.16]

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.

Backport of !5756 (merged)