BUG: numpydoc/linkcode: do not detect linkcode config changes

Since the linkcode_resolve is a function, it triggers configuration
change on each rebuild, resulting to full rebuild always. It's better
to make it not do that.

doc/sphinxext/linkcode.py

@@ -76,4 +76,4 @@ def doctree_read(app, doctree):
 
 def setup(app):
     app.connect('doctree-read', doctree_read)
-    app.add_config_value('linkcode_resolve', None, 'env')
+    app.add_config_value('linkcode_resolve', None, '')