azurerm_cosmosdb_mongo_collection: remove _id index check

[wuxu92]

This is caused by a living API Breaking change, so this PR is waiting for service team's fix

the _id index was not required anymore, we cannot set it even or the API will raise error: Message="The field 'unique' is not valid for _id index specification.\r\nActivityId.

document: https://learn.microsoft.com/en-us/azure/cosmos-db/mongodb/indexing#indexing-for-mongodb-server-version-36-and-higher

[stephybun]

@wuxu92 is there a timeline for when the service team will release their fix for this?

[wuxu92]

@stephybun It appears that the service team has reversed the breaking change that caused this issue. The AccTest has been successfully passed for some time now, so I will close this PR.

[github-actions]

I'm going to lock this pull request because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active contributions.
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