Simplify the AttributeOutputBase::OutputFlyFrame method.

Change-Id: Iac3b1b627728d5268ce7633904af9113e3127be2
Reviewed-on: https://gerrit.libreoffice.org/9006
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>