remove unsigned SBOM section from SBOM tutorial

[imjasonh]

Cosign complains when attaching unsigned SBOMs, and we shouldn't recommend this path when signed SBOMs are better.

WARNING: SBOM attachments are deprecated and support will be removed in a Cosign release soon after 2024-02-22 (see https://github.com/sigstore/cosign/issues/2755). Instead, please use SBOM attestations.
WARNING: Attaching SBOMs this way does not sign them. To sign them, use 'cosign attest --predicate amd_64.spdx --key <key path>'.

Type of change

deletion

What should this PR do?

remove the section about unsigned SBOMs

Why are we making this change?

we shouldn't recommend unsigned SBOMs, since Cosign complains about them, and signed SBOMs are better.

What are the acceptance criteria?

clarity and flow

How should this PR be tested?

n/a, nothing to test

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