It is a misnomer now, so return to the original name.
Also drop the redundant private Font::GetAverageFontSize().
Change-Id: I67495fa675091b459a4e90ac09665489cc61fdd9
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/207991
Tested-by: Jenkins
Reviewed-by: Khaled Hosny <khaled@libreoffice.org>