fdo#75757: remove inheritance to std::vector

from FontPortionModelList.

Change-Id: Ice34808107a7b381e39d5f7d164590b48a630ee0
Reviewed-on: https://gerrit.libreoffice.org/11229
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>