Remove orphaned indices for a namespace

What does this MR do and why?

Remove orphaned indices for a namespace

Ultimately, we plan to use the index state field to filter out orphaned indices, but there is an edge case where the associated enabled namespace has been deleted but the state has not been updated yet. This added filter will protect us from that scenario moving forward.

Query plan before: https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/27094/commands/84258

Query plan after: https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/27094/commands/84257

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Before	After

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