Do not set html_last_updated_fmt

Since version 1.20 of openstackdocstheme, html_last_updated_fmt does not
need to be set anymore, remove it's setting.

Change-Id: I913b76cc6b3128a420a6fc69e2dc88627d47bbfe

api-ref/source/conf.py

@@ -156,8 +156,3 @@ html_theme_options = {"sidebar_dropdown": "api_ref",
 # If not '', a 'Last updated on:' timestamp is inserted at every page bottom,
 # using the given strftime format.
 #html_last_updated_fmt = '%b %d, %Y'
-
-# So that we can enable "log-a-bug" links from each output HTML page, this
-# variable must be set to a format that includes year, month, day, hours and
-# minutes.
-html_last_updated_fmt = '%Y-%m-%d %H:%M'

releasenotes/source/conf.py

@@ -146,7 +146,6 @@ html_static_path = ['_static']
 # instead of the build time.
 # See https://reproducible-builds.org/specs/source-date-epoch/
 # html_last_updated_fmt = '%b %d, %Y'
-html_last_updated_fmt = '%Y-%m-%d %H:%M'
 
 # If true, SmartyPants will be used to convert quotes and dashes to
 # typographically correct entities.