swan: Remove tag definition for http proxy

The tag is being removed, so that the chart uses the one that is set by
default in the default values of the jupyterhub chart, which may change
in different versions.

This prevents the tag not being updated in our chart if we forget.

swan/values.yaml

@@ -92,7 +92,6 @@ jupyterhub:
           privateIPs: false
       image:
         name: "registry.cern.ch/quay.io/jupyterhub/configurable-http-proxy"
-        tag: "4.5.0"
         pullPolicy: "IfNotPresent"
       resources:
         requests: