Remove Sphinx conf html_static_path to missing directory

When building the docs, fixes the warning:

  html_static_path entry '.../django-storages/docs/_static' does not exist

docs/conf.py

@@ -121,7 +121,7 @@
 # Add any paths that contain custom static files (such as style sheets) here,
 # relative to this directory. They are copied after the builtin static files,
 # so a file named "default.css" will overwrite the builtin "default.css".
-html_static_path = ['_static']
+#html_static_path = ['_static']
 
 # If not '', a 'Last updated on:' timestamp is inserted at every page bottom,
 # using the given strftime format.