Remove unsupported stackdriver tracing backend

[lionelvillard]

knative/pkg#2183 followup.

Proposed Changes

• Remove mentions of stackdriver in config-tracing.

Pre-review Checklist

• At least 80% unit test coverage
• E2E tests for any new behavior
• Docs PR for any user-facing impact
• Spec PR for any new API feature
• Conformance test for any change to the spec

Release Note

Docs

[codecov]

Codecov Report

Merging #5881 (f61b1a7) into main (81afe0b) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #5881   +/-   ##
=======================================
  Coverage   81.96%   81.96%           
=======================================
  Files         220      220           
  Lines        7458     7458           
=======================================
  Hits         6113     6113           
  Misses        918      918           
  Partials      427      427           

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[odacremolbap]

+1
although the original issue also mentions dropping Stackdriver metrics.
I guess that one is left for a different PR.

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: lionelvillard, odacremolbap

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