fix: dev: Fix a bug in qpzone.c:first_existing_header_indirect()

There is a bug in qpzone.c:first_existing_header_indirect() where
it does not advance the pointer in the FOREACH type loop.

Remove the static function altogether, as it was used only once and
had some other problems too, and use simpler custom code instead in
the place where it was used.

Closes #5691

Merge branch '5691-qpzone.c-first_existing_header_indirect-fix' into 'main'

See merge request isc-projects/bind9!11460