fix(manifests): revert manifest feat flag for disabling failing evaluations

[zachsmith1]

This reverts commit 786a365.

[spinnakerbot]

The following commits need their title changed:

• 2d9b602: Revert "feat(manifests): add property to make failing on unknown manifest variables optional (feat(manifests): add property to make failing on unknown manifest variables optional #3800)"

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[kevinawoo]

discussed in k8s sig to remove it in favor of change the PCF UI to support this, 👍