doc: Clear DOT_FONTNAME

doxygen now produces a warning, let's use the default instead.

doc/Doxyfile.in

@@ -1719,7 +1719,7 @@ DOT_NUM_THREADS        = 0
 # the DOTFONTPATH environment variable or by setting DOT_FONTPATH to the
 # directory containing the font.
 
-DOT_FONTNAME           = FreeSans
+DOT_FONTNAME           =
 
 # The DOT_FONTSIZE tag can be used to set the size of the font of dot graphs.
 # The default size is 10pt.