doc/conf.py: do not set html_use_smartypants explicitly

doc/conf.py: do not set html_use_smartypants explicitly

because it's deprecated, and is on by default in the specified sphinx
version (v1.6.3):

this change silences following warning:

build-doc/virtualenv/local/lib/python2.7/site-packages/sphinx/environment/__init__.py:683:
RemovedInSphinx17Warning: html_use_smartypants option is deprecated.
Smart quotes are on by default; if you want to disable or customize
them, use the smart_quotes option in docutils.conf.
  RemovedInSphinx17Warning)

Signed-off-by: Kefu Chai <kchai@redhat.com>