Drop font substitution tab page

The relevant VCL code will be dropped in the next commits.

Change-Id: Ieaa550d671fa43f2e5a96e2df7f256de2faf2fa4
Reviewed-on: https://gerrit.libreoffice.org/6857
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>