remove outdated comment (#3079)

rxd hasn't used scipy in years, so the comment is inaccurate. However, keeping track of contours is not inherently related to rxd, so I'm not sure that there is a better place for this dictionary than at the top level.

share/lib/python/neuron/__init__.py

@@ -748,8 +748,6 @@ def _modelview_mechanism_docstrings(dmech, tree):
                 tree.append(line, dmech.location, 0)
 
 
-# TODO: put this someplace else
-#       can't be in rxd because that would break things if no scipy
 _sec_db = {}