remove unused _static path that triggers warning

docs/source/conf.py

@@ -93,7 +93,7 @@ html_theme = 'sphinx_rtd_theme'
 # 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 = []
 
 # Both the class’ and the __init__ method’s docstring
 # are concatenated and inserted.