DOC: remove dead "Show Source" links

Remove dead links to source code as _source directory was intentionally
removed in the circle config config.yml because store_artifcats uploads
files one by one which takes an inordinate amount of time.

Author: StefRe

doc/conf.py

@@ -381,7 +381,8 @@ def js_tag_with_cache_busting(js):
     "logo": {"link": "index",
              "image_light": "images/logo2.svg",
              "image_dark": "images/logo_dark.svg"},
-    "navbar_end": ["theme-switcher", "version-switcher", "mpl_icon_links"]
+    "navbar_end": ["theme-switcher", "version-switcher", "mpl_icon_links"],
+    "page_sidebar_items": "page-toc.html",
 }
 include_analytics = is_release_build
 if include_analytics: