leap: stop pulling the tensorflow image (slows down other startups)

This was an optimization for a workshop where I assume nodes were
pre-started and made sense for that, but would slow down startup on new
nodes if not this specific image was requested on a non-pre-warmed node
after the workshop.

Since the workshop has ended, we can safely remove this now.

Related tickets:
- Workshop: https://2i2c.freshdesk.com/a/tickets/349
- Slow startup discussed: https://2i2c.freshdesk.com/a/tickets/435

config/clusters/leap/common.values.yaml

@@ -13,11 +13,6 @@ basehub:
     prePuller:
       continuous:
         enabled: true
-      # Extra images to be pulled on all nodes
-      extraImages:
-        tensorflow-image:
-          name: pangeo/ml-notebook
-          tag: "ebeb9dd"
     custom:
       # Extra mount point for admins to access to all users' home dirs
       # Ref https://github.com/2i2c-org/infrastructure/issues/2105