doxygen: Do not use FreeSans as dot fontname

The FreeSans font no longer ships with Doxygen and might not be available.

doc/doxygen.cfg.in

@@ -2071,7 +2071,7 @@ DOT_NUM_THREADS        = 0
 # The default value is: Helvetica.
 # This tag requires that the tag HAVE_DOT is set to YES.
 
-DOT_FONTNAME           = FreeSans
+DOT_FONTNAME           =
 
 # The DOT_FONTSIZE tag can be used to set the size (in points) of the font of
 # dot graphs.