Remove mkdocs-material-extensions

[gcs278]

What type of PR is this?
/kind cleanup

What this PR does / why we need it:
The mkdocs-material-extensions package is now deprecated, and it's usage in mkdocs.yaml was removed in #3663. This fix also removes it from requirements.txt to prevent unnecessary installation.

Which issue(s) this PR fixes:
N/A

Does this PR introduce a user-facing change?:

NONE

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: gcs278
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[k8s-ci-robot]

@gcs278: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-gateway-api-test	b60e543	link	true	/test pull-gateway-api-test
pull-gateway-api-crds-validation-5	b60e543	link	true	/test pull-gateway-api-crds-validation-5
pull-gateway-api-verify	b60e543	link	true	/test pull-gateway-api-verify
pull-gateway-api-crds-validation-3	b60e543	link	true	/test pull-gateway-api-crds-validation-3
pull-gateway-api-crds-validation-4	b60e543	link	true	/test pull-gateway-api-crds-validation-4

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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[gcs278]

oh sorry, #3663 already took care of this
/close

[k8s-ci-robot]

@gcs278: Closed this PR.

In response to this:

oh sorry, #3663 already took care of this
/close

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[blake]

@gcs278 I actually overlooked removing this from the requirements file in my PR. This is still a valid change IMO. It never hurts to clean up unused dependencies.

[gcs278]

@blake Oh good point, I overlooked the requirements.txt fix I made.
/open

[gcs278]

/reopen

[k8s-ci-robot]

@gcs278: Failed to re-open PR: state cannot be changed. The remove-mkdocs-material-extensions branch was force-pushed or recreated.

In response to this:

/reopen

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[gcs278]

Oh darn I locked myself out from this PR by force pushing it while it was closed.

Created #3666