Remove clock import from RotatingBoxes.ipynb

Author: BruceSherwood

Demos/RotatingBoxes.ipynb

@@ -107,7 +107,6 @@
    "source": [
     "from vpython import *\n",
     "scene = canvas() # This is needed in Jupyter notebook and lab to make programs easily rerunnable\n",
-    "from time import clock\n",
     "\n",
     "N = 10\n",
     "\n",