Use session.delete rather than kernel.kill

IPython/html/static/notebook/js/notebook.js

@@ -258,7 +258,7 b' define(['
             // TODO: Make killing the kernel configurable.
             var kill_kernel = false;
             if (kill_kernel) {
-                that.kernel.kill();
+                that.session.delete();
             }
             // if we are autosaving, trigger an autosave on nav-away.
             // still warn, because if we don't the autosave may fail.