doc: remove obsolete HTML_TIMESTAMP

warning: Tag 'HTML_TIMESTAMP' at line 44 of file '/home/whot/xorg/lib/libxkbcommon/build/Doxyfile' has become obsolete.

Author: whot

doc/Doxyfile.in

@@ -41,7 +41,6 @@ IGNORE_PREFIX          = xkb_ \
 HTML_EXTRA_STYLESHEET  = doc/doxygen-extra.css
 
 TIMESTAMP              = NO
-HTML_TIMESTAMP         = NO
 
 ENUM_VALUES_PER_LINE   = 1