Remove dask gateway example

[jsignell]

On the default pangeo image and also on newer pangeo image dask gateway never successfully scales the cluster. It seems like the scale request never really gets through to the cluster. I wrote this up in NASA-IMPACT/veda-jupyterhub#105

This proposal on the veda-docs side is to remove dask-gateway from the docs. I think we can just point to the 2i2c dask gateway docs anyways to cover the concept.

Originally posted by @jsignell in NASA-IMPACT/pangeo-notebook-veda-image#54 (comment)

[wildintellect]

• Dask gateway doesn't create workers veda-jupyterhub#105

I'm unclear after reading the 2i2c tickets - is this resolved now, no action required?

[jsignell]

Right! It's resolved on jupyterhub now.