gnu: Remove python2-scikit-image.

Some of the packages that this depends upon no longer supports Python 2.

* gnu/packages/python-science.scm (python2-scikit-image): Remove variable.

gnu/packages/python-science.scm

@@ -195,9 +195,6 @@ routines such as routines for numerical integration and optimization.")
      "Scikit-image is a collection of algorithms for image processing.")
     (license license:bsd-3)))
 
-(define-public python2-scikit-image
-  (package-with-python2 python-scikit-image))
-
 (define-public python-pandas
   (package
     (name "python-pandas")