src/sage/repl/rich_output/backend_ipython.py: no native Windows paths

We're dropping Cygwin support, and in particular, the pycygwin library
that handles these paths.

Author: orlitzky

src/sage/repl/rich_output/backend_ipython.py

@@ -419,13 +419,6 @@ def threejs_offline_scripts(self):
 
         script = os.path.join(THREEJS_DIR, '{}/three.min.js'.format(_required_threejs_version()))
 
-        if sys.platform == 'cygwin':
-            import cygwin
-
-            def normpath(p):
-                return 'file:///' + cygwin.cygpath(p, 'w').replace('\\', '/')
-            script = normpath(script)
-
         return '\n<script src="{0}"></script>'.format(script)