Drop incubating theme from docs

OpenStack has dropped the incubation notion, so labeling this as an
incubated project in the docs is confusing.

Change-Id: I5bc1e238c2993903636aa60ddd400f9c68c557b1

doc/source/conf.py

@@ -58,7 +58,7 @@ pygments_style = 'sphinx'
 # html_theme_path = ["."]
 # html_theme = '_theme'
 # html_static_path = ['static']
-html_theme_options = {'incubating': True}
+html_theme_options = {}
 
 # Output file base name for HTML help builder.
 htmlhelp_basename = '%sdoc' % project