drop unnecessary external name svc

[dprotaso]

When debugging the visibility split issues in knative-extensions/net-contour#1149 I realize we don't need the extra ExternalName K8s Service.

It was being used to supply a DNS entry that would map a host name to the contour internal envoy service.

As an alternative we can add a GATEWAY_HOST env var that will configure the conformance httpproxy app to just dial the envoy service directly.

[knative-prow]

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[dprotaso]

note: I tested the change in a wip commit here: knative-extensions/net-contour@c0ebfc8

it passed conformance tests here: https://prow.knative.dev/view/gs/knative-prow/pr-logs/pull/knative-extensions_net-contour/1149/integration-tests_net-contour_main/1877181980768997376

[dprotaso]

/assign @skonto

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.14%. Comparing base (34f5e09) to head (67e5a22).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1029   +/-   ##
=======================================
  Coverage   93.14%   93.14%           
=======================================
  Files          36       36           
  Lines        1254     1254           
=======================================
  Hits         1168     1168           
  Misses         72       72           
  Partials       14       14           

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