conf.py: remove display_version configuration

the option for display_version for the sphinx-rtd-theme
was deprecated since v3.0.0

Signed-off-by: Christopher Turner <christopher.g.turner@intel.com>

Author: cgturner1

conf.py

@@ -136,7 +136,6 @@
         'canonical_url': '',
         'analytics_id': 'GTM-M4BL5NF',
         'logo_only': False,
-        'display_version': True,
         'prev_next_buttons_location': 'None',
         # Toc options
         'collapse_navigation': False,