Remove reference to non-existent directory containing static files to fix warning when building developer documentation (#14162)
related to #12971
Summary of the issue:
When building developer documentation the following warning is shown:
WARNING: html_static_path entry '_static' does not exist
Description of user facing changes
One less warning when building developer documentation is shown.
Description of development approach
Since we don't have any custom static files I've just removed the path - that was a left over from the default Sphinx template.
Testing strategy:
Created developer documentation made sure that this specific warning is gone.