Bug 34711: Remove use of onclick for opac-privacy.pl

Bug 34711: Remove use of onclick for opac-privacy.pl

This change removes the "onclick" attribute for the buttons
on opac-privacy.pl.

Note: The Javascript is placed in a separate file, which is
the preferred way for Javascript to be handled by
Content-Security-Policy

Test plan:
0. Apply the patch
1. Go to http://localhost:8081/cgi-bin/koha/admin/preferences.pl?op=search&searchfield=OPACPrivacy
2. Set syspref to "Allow"
3. Go to http://localhost:8080/cgi-bin/koha/opac-privacy.pl
4. The confirmation modal appears and "OK" and "Cancel" buttons work as
   expected

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>