Remove local-gateway.mesh: "mesh" option

[nak3]

As per knative/serving#11523, this patch removes local-gateway.mesh: "mesh".

Please note that it does not remove local-gateway.<GATEWAY>: <GATEWAY_SVC> is still availble.
This patch removes only mesh which is a special configuration.

Release Note

`local-gateway.mesh: "mesh"` option was dropped.

/cc @ZhiminXiang @carlisia

[codecov]

Codecov Report

Merging #915 (d223205) into main (3295aa6) will decrease coverage by 0.10%.
The diff coverage is n/a.

❗ Current head d223205 differs from pull request most recent head cfa4a7d. Consider uploading reports for the commit cfa4a7d to get more accurate results

@@            Coverage Diff             @@
##             main     #915      +/-   ##
==========================================
- Coverage   80.95%   80.85%   -0.11%     
==========================================
  Files          18       18              
  Lines        1607     1598       -9     
==========================================
- Hits         1301     1292       -9     
  Misses        223      223              
  Partials       83       83              

Impacted Files	Coverage Δ
pkg/reconciler/ingress/config/istio.go	100.00% <ø> (ø)

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[carlisia]

/retest

[carlisia]

/lgtm
/approve

[knative-prow]

[APPROVALNOTIFIER] This PR is APPROVED

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