Remove useless overrides in vcl strhelper

Change-Id: I85996380ea5b3a157ba573182da0bc380effa1e5
Reviewed-on: https://gerrit.libreoffice.org/13464
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>