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
Don't forget to remove it from the about page when we are done here.