web: Clear DOT_FONTNAME in Doxyfile.

Doxygen-1.8.6. warns about DOT_FONTNAME being set:

	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.

The warning is harmless because the DOT_FONTNAME option has no effect
in our setup because HAVE_DOT is set to "NO". Setting DOT_FONTNAME
to the empty string, as recommended in the explanatory text after
the warning, silences the warning.