Remove Colab links in notebooks. Remove old environment.yml file.

[leehart]

Re: issue #94

Also issue #83

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[leehart]

We will need to check to see if this actually deploys OK, after it gets merged into master and triggers the GitHub Actions.

jupyter-book/jupyter-book#1428 suggests that there will still be a rocket icon, but it will display an empty dropdown, which we don't' have a solution for yet.

[ahernank]

Fyi, code from training repo:

launch_buttons:
  notebook_interface: "jupyterlab"
  colab_url: "https://colab.research.google.com"
  binderhub_url: ""  # disable binderhub, support colab only

[ahernank]

Thanks @leehart! Seems kind of fair that if we want to fully clean-up the "interactiveness", i.e. the rocket, to have to go full markdown.