remove special logic for user-agent header with istio mtls

[izabelacg]

Changes

• Remove logic that adds a custom header (k-kubelet-probe) in requests coming from kubelet with istio mtls

Fixes #14981

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.06%. Comparing base (932ef55) to head (dd4e973).
Report is 13 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15034      +/-   ##
==========================================
- Coverage   84.07%   84.06%   -0.02%     
==========================================
  Files         213      213              
  Lines       16796    16776      -20     
==========================================
- Hits        14121    14102      -19     
- Misses       2320     2322       +2     
+ Partials      355      352       -3     

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[izabelacg]

/restest

[izabelacg]

The following PR has the required changes also in networking, pkg and net-istio: #15011. The only failed tests in that PR are tests that currently fail in main as well.

[izabelacg]

/retest

[izabelacg]

/retest

[dprotaso]

/lgtm
/approve

[knative-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dprotaso, izabelacg

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