Bug 34570: Remove use of "onclick" for PopupMARCFieldDoc()

Bug 34570: Remove use of "onclick" for PopupMARCFieldDoc()

This change removes the "onclick" attribute for the MARC doc
link in the cataloguing editor.

Test plan:
0. Apply patch
1. Go to http://localhost:8081/cgi-bin/koha/cataloguing/addbiblio.pl?frameworkcode=#
2. Click on the question mark (?) next to any of the MARC fields
3. Press the "Repeat this tag" button
4. Click on the question mark (?) on the cloned MARC field
5. Note that both times it worked

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>