Using a timestamp as value for "html_last_updated_fmt" isn't good
for the reproducible builds initiative from different downstream
distributions because the build result always differs due to the
timestamp in the documentation.
Also when not setting "html_last_updated_fmt", the value can be set
via command line when running "sphinx-build -D".
Change-Id: Ic0c6dfc27a44d9b48e9a0c643f6301f596074c87