fix: 'openstack-tox-docs' gate

The 'openstack-tox-docs' gate is currently broken:
`html_static_path entry '_static' does not exist`

Removed the 'html_static_path' from 'doc/source/conf.py' as this is
also how Tempest is configured [0]

[0] https://github.com/openstack/tempest/blob/master/doc/source/conf.py#L125

Change-Id: I1244457428cbefb9341a1991839d41b592b6c469
diff --git a/doc/source/conf.py b/doc/source/conf.py
index 57d763e..1d82bc0 100755
--- a/doc/source/conf.py
+++ b/doc/source/conf.py
@@ -81,7 +81,6 @@
 # The theme to use for HTML and HTML Help pages.  Major themes that come with
 # Sphinx are currently 'default' and 'sphinxdoc'.
 # html_theme_path = ["."]
-html_static_path = ['_static']
 html_theme = 'openstackdocs'
 
 # openstackdocstheme options