Remove duplicate gcpbackendpolicy and healthcheckpolicy config

[liu-cong]

What type of PR is this?
/kind bug

What this PR does / why we need it:
This PR fix 2 bugs:

EDIT: The first bug is already fixed by #1600

1. Add monitoring.gke.enabled field to values.yaml, otherwise helm apply will fail with

Error: UPGRADE FAILED: template: inferencepool/templates/epp-sa-token-secret.yaml:1:74: executing "inferencepool/templates/epp-sa-token-secret.yaml" at <.Values.inferenceExtension.monitoring.gke.enabled>: nil pointer evaluating interface {}.enabled

2. Remove the healthcheckpolicy and gcpbackendpolicy from the static manifest. These are already added to the helm chart with provider=gke. Currently following the user guide will result in duplicated healthcheckpolicy and gcpbackendpolicy for GKE, and users need to manually dedupe for gateway to function.

Which issue(s) this PR fixes:

Fixes #

Does this PR introduce a user-facing change?:

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[liu-cong]

@kfswain Another bug fix for the patch.

[ahg-g]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ahg-g, liu-cong

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[ahg-g]

@kfswain Another bug fix for the patch.

This doesn't require a patch since those manifests are not part of the release.

[kfswain]

@kfswain Another bug fix for the patch.

This doesn't require a patch since those manifests are not part of the release.

++