don't use fixed size icon sets, fdo#62652

actually we should switch to svg as soon as it becomes possible

Change-Id: Ifbbe5befab94441f04fadb90b779e0bc8c15eda4
Reviewed-on: https://gerrit.libreoffice.org/6890
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>