When I grep for "greybox" in the OPAC, I only really see one two relevant references: koha-tmpl/opac-tmpl/bootstrap/en/includes/greybox.inc koha-tmpl/opac-tmpl/bootstrap/en/modules/opac-detail.tt The "opac-detail.tt" is just including "greybox.inc" I don't know enough about greybox to know how to remove it but it looks like it should be easy enough?
See Bug 32333