cleanup: remove stale FIXME in terraform provider

[AndrewCharlesHay]

Removes a stale FIXME comment in internal/builtin/providers/terraform/provider.go. The suggested validation logic is already present in dataSourceRemoteStateValidate.

Connection to OpenTofu

This stale FIXME was also identified in the OpenTofu fork. An identical cleanup has been submitted to OpenTofu in opentofu/opentofu#3625.

Timeline:

1. July 2018: The FIXME was introduced in builtin/providers: terraform remote state datasource #18446 when the provider was created.
2. May 2019: The validation logic was implemented in providers/terraform: Explicit validate step #21205, but the FIXME comment was accidentally left behind.
3. Dec 2025: This PR removes the stale comment.

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[radeksimko]

Thanks for the catch. Would you mind removing the changelog entry (as this change is not user-facing) and signing the CLA?

Also, for anyone else reading this in the future, the PR which implemented the validation is #21205 (different from the one mentioned, probably by mistake)

[AndrewCharlesHay]

I updated PR description and signed the CLA and removed the changelog entry