gnu: Remove python2-notebook.

Some of its requirements no longer support Python 2 which has left this
package broken for a while.

* gnu/packages/python-xyz.scm (python2-notebook): Remove variable.
(python-notebook)[properties]: Remove.

gnu/packages/python-xyz.scm

@@ -11200,29 +11200,8 @@ convert an @code{.ipynb} notebook file into various static formats including:
     (description
      "The Jupyter HTML notebook is a web-based notebook environment for
 interactive computing.")
-    (properties `((python2-variant . ,(delay python2-notebook))))
     (license license:bsd-3)))
 
-(define-public python2-notebook
-  (let ((base (package-with-python2
-                (strip-python2-variant python-notebook))))
-    (package/inherit base
-      (native-inputs
-       `(("python2-mock" ,python2-mock)
-         ,@(package-native-inputs base)))
-      (arguments
-       (substitute-keyword-arguments (package-arguments base)
-         ((#:phases phases)
-          `(modify-phases ,phases
-             (add-before 'check 'disable-test-case
-              ;; The test requires network access to localhost. Curiously it
-              ;; fails with Python 2 only. Simply make the test-case return
-              ;; immediately.
-              (lambda _
-                (substitute*
-                    "notebook/services/nbconvert/tests/test_nbconvert_api.py"
-                  (("formats = self.nbconvert_api") "return #")))))))))))
-
 (define-public python-widgetsnbextension
   (package
     (name "python-widgetsnbextension")