[maint] Clean up doxyfile

[jeromekelleher]

Looks like there's some stuff in the Doxyfile that needs cleaning up:

warning: ignoring unsupported tag `OUTPUT_TEXT_DIRECTION  =' at line 102, file Doxyfile
warning: ignoring unsupported tag `JAVADOC_BANNER         =' at line 208, file Doxyfile
warning: ignoring unsupported tag `OPTIMIZE_OUTPUT_SLICE  =' at line 307, file Doxyfile
warning: ignoring unsupported tag `EXTRACT_PRIV_VIRTUAL   =' at line 483, file Doxyfile
warning: ignoring unsupported tag `HTML_DYNAMIC_MENUS     =' at line 1233, file Doxyfile
warning: ignoring unsupported tag `LATEX_MAKEINDEX_CMD    =' at line 1698, file Doxyfile
warning: ignoring unsupported tag `LATEX_EMOJI_DIRECTORY  =' at line 1840, file Doxyfile
warning: ignoring unsupported tag `XML_NS_MEMB_FILE_SCOPE =' at line 1984, file Doxyfile