Remove the proof-of-concept shell script that post-processes the
generated HTML to add the git date when the corresponding .rst file as
the last modified date, augmenting the existing published date.
Instead, use a custom last_updated.py Sphinx extension that's integrated
into the Sphinx build itself. Remove the old fix-git-modified-date.sh
script and calls to it.
Signed-off-by: David B. Kinder <david.b.kinder@intel.com>
