Fix warning in build while copying static files

Based on readthedocs/readthedocs.org#1776 this might
do the trick

Refs SHOOP-2419

Author: tulimaki

source/conf.py

@@ -139,7 +139,7 @@
 # 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 = []
 
 # Add any extra paths that contain custom files (such as robots.txt or
 # .htaccess) here, relative to this directory. These files are copied