DOC: Remove inheritance-diagram Sphinx role.

Either the role should be removed (as done here) or the inheritance-diagram extension needs to be added to the config file. Since skimage doesn't have a complicated class hierarchies, it's probably best to remove.

doc/tools/apigen.py

@@ -281,11 +281,6 @@ class ApiDocWriter(object):
             title = ':mod:`' + uri_short + '`'
         ad += title + '\n' + self.rst_section_levels[1] * len(title)
 
-        if len(classes):
-            ad += '\nInheritance diagram for ``%s``:\n\n' % uri
-            ad += '.. inheritance-diagram:: %s \n' % uri
-            ad += '   :parts: 3\n'
-
         ad += '\n.. automodule:: ' + uri + '\n'
         ad += '\n.. currentmodule:: ' + uri + '\n'
 #        multi_class = len(classes) > 1