Bug 29897: Don't use $6 for linkage but build the link instead

Bug 29897: Don't use $6 for linkage but build the link instead

See comment 26:
$6 is a very specialized field that will never contain a URL. Linkage here means 'linking MARC fields within the record'. For example it is used to link an original script tag with the corresponding tag in transliteration. See: https://www.loc.gov/marc/authority/ecadcntf.html

Sponsored-by: Orex Digital
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>