Remove deprecated doxygen flags

Remove deprecated doxygen flags

Running 'doxygen -u Doxyfile.in' in the source root produces the
following results:

 - SYMBOL_CACHE_SIZE at line 301 has become obsolete. This tag
   has been removed.
 - SHOW_DIRECTORIES at line 507 has become obsolete. This tag
   has been removed.
 - HTML_ALIGN_MEMBERS at line 881 has become obsolete. This tag
   has been removed.
 - USE_INLINE_TREES at line 1067 has become obsolete. This tag
   has been removed.
 - XML_SCHEMA at line 1311 has become obsolete. This tag has been
   removed.
 - XML_DTD at line 1317 has become obsolete. This tag has been
   removed.

Signed-off-by: Richard B Winters <rik@mmogp.com>
Reviewed-by: Jan Friesse <jfriesse@redhat.com>