remove unused code from TabBar

Change-Id: I9f975d99893135b414c749c4fe86912362375c4e
Reviewed-on: https://gerrit.libreoffice.org/9162
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>