Remove obsolete config switches.

Fixes:
Warning: Tag `SYMBOL_CACHE_SIZE' at line 290 of file /mnt/vdb/lsf/cling-37f64ff95641-full-1498388/obj/tools/cling/docs/doxygen.cfg has become obsolete.
To avoid this warning please remove this line from your configuration file or upgrade it using "doxygen -u"
Warning: Tag `SHOW_DIRECTORIES' at line 490 of file /mnt/vdb/lsf/cling-37f64ff95641-full-1498388/obj/tools/cling/docs/doxygen.cfg has become obsolete.
To avoid this warning please remove this line from your configuration file or upgrade it using "doxygen -u"
Warning: Tag `HTML_ALIGN_MEMBERS' at line 819 of file /mnt/vdb/lsf/cling-37f64ff95641-full-1498388/obj/tools/cling/docs/doxygen.cfg has become obsolete.
To avoid this warning please remove this line from your configuration file or upgrade it using "doxygen -u"
Warning: Tag `USE_INLINE_TREES' at line 989 of file /mnt/vdb/lsf/cling-37f64ff95641-full-1498388/obj/tools/cling/docs/doxygen.cfg has become obsolete.
To avoid this warning please remove this line from your configuration file or upgrade it using "doxygen -u"
Warning: doxygen no longer ships with the FreeSans font.
You may want to clear or change DOT_FONTNAME.
Rely on the expr kind, instead of some compiler generated exprs which can be missing.