doc: clean ENABLED_SECTION option for doxygen

The ENABLED_SECTION option is not a YES/NO option but should
contain a list of section label to be processed. For example,
adding INTERNAL_HIDDEN will make doxygen to look through all
the blocks with "@cond INTERNAL_HIDDEN" and "@endcond". So
just remove the "YES" and leave it an empty list.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>