remove dask and conda config files from base image

[scottyhq]

closes #94

dask config will need to be set per image (or in hub environment variables going forward):

In particular:

distributed:
  dashboard:
    link: /user/{JUPYTERHUB_USER}/proxy/{port}/status

labextension: 
  factory: 
    module: dask_gateway 
    class: GatewayCluster 
    args: [] 
    kwargs: {} 

[pangeo-bot]

/condalock
Automatically locking new conda environment, building, and testing images...

[scottyhq]

@TomAugspurger - i've confirmed that images build and run as expected locally with these changes. i'm thinking we go ahead and merge to more easily test on binder and staging hubs. thoughts?