docs: drop removed OpaqueProvider from AttestationUnsupported row

[imran-siddique]

#236 removed OpaqueProvider, but the AttestationUnsupported row in docs/spec/error-codes.md still listed it as a source of the error. This drops that clause. The remaining text matches the code: AttestationUnsupported is raised by a provider's attest when the host lacks what its collector needs (tee/base.py, tee/sev_snp.py) and by the hardware acceptance run.

Docs only, one line.

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[carloshvp]

Reviewed exact head a24b1e9370f8825de08853fc42ed684a3567ef8c in an isolated checkout. Removing the OpaqueProvider clause matches the implementation: that provider is absent from src, while the active TPM, SEV-SNP, TDX/configfs-TSM providers and hardware-acceptance path raise AttestationUnsupported when the required collection capability is unavailable. BaseProvider documents the same behavior.

The diff removes only the obsolete clause. The status, detail guidance and existing Peer Attestation link are preserved, and the linked document exists. The commit has the required Signed-off-by trailer. The CR-at-EOL-aware diff check and merge simulation against fetched main passed; hosted checks are successful or expected skips. Approving.