doc: Remove unsupported font name
doc: Remove unsupported font name

Fix this warning from doxygen:

    warning: doxygen no longer ships with the FreeSans font.
    You may want to clear or change DOT_FONTNAME.
    Otherwise you run the risk that the wrong font is being used for dot generated graphs.

Signed-off-by: Stefan Weil <8d4c780fcfdc41841e5070f4c43da8958ba6aec0@weilnetz.de>

amitdo/tesseract,UB-Mannheim/tesseract,amitdo/tesseract,tesseract-ocr/tesseract,amitdo/tesseract,tesseract-ocr/tesseract,amitdo/tesseract,jbarlow83/tesseract,tesseract-ocr/tesseract,jbarlow83/tesseract,UB-Mannheim/tesseract,amitdo/tesseract,tesseract-ocr/tesseract,stweil/tesseract,UB-Mannheim/tesseract,stweil/tesseract,jbarlow83/tesseract,UB-Mannheim/tesseract,tesseract-ocr/tesseract,stweil/tesseract,jbarlow83/tesseract,jbarlow83/tesseract,UB-Mannheim/tesseract,stweil/tesseract,stweil/tesseract