Fix another warning about fonts for DOT

DOT_FONTNAME was set to FreeSans, which is not shipped with Doxygen
anymore. According to the documentation, "his tag requires that the tag
HAVE_DOT is set to YES." Since we have it to NO, there is no problem in
removing DOT_FONTNAME.
Merge pull request #12 from akfidjeland/FixWarning

Remove SYMBOL_CACHE_SIZE to avoid warning