remove jupyter_contrib_nbextensions

In the environment.yml. Causes the image not to build.

Also I think the jupyter-singleuser line in environment.yml is out of date.

Author: eeholmes

environment.yml

@@ -4,7 +4,6 @@ channels:
   - conda-forge
 
 dependencies:
-  - jupyter_contrib_nbextensions==0.5.1
   # Required until https://github.com/jupyterhub/repo2docker/pull/1196 is merged
   - jupyterhub-singleuser>=3.0,<4.0
   # Set default python version to 3.10 - repo2docker sets it to 3.7 instead by default,