Remove unused former source of tensorflow.org/tutorials/image_retraining.

The source of https://tensorflow.org/tutorials/image_retraining has moved from
https://github.com/tensorflow/tensorflow/tree/master/tensorflow/docs_src/tutorials
to https://github.com/tensorflow/hub/tree/master/docs/tutorials
because of its use of TensorFlow Hub.

This change replaces the now-defunct version with a pointer to the new
location, in order to avoid dead code.

PiperOrigin-RevId: 192451570