Remove hands-on material

Although a link to Google Colab is a good idea, the data is not
on Google Colab, making executing the notebook there impossible.
The notebook could be adapted to deal with that, but for now, it
is better to remove it.

Author: gjbex

README.md

@@ -9,7 +9,6 @@ For information on the training, see the website
 
 1. [`python_for_data_science.pptx`](python_for_data_science.pptx): PowerPoint
    presentation used for the training.
-1. [`hands-on`](hands-on): Jupyter notebooks for hands-on sessions.
 1. [`source-code`](source-code): sample code written to develop the slides and
    illustrate concepts.
 1. [`environment.yml`](environment.yml): conda environment file intended to be