fix(sbom): scan results of SBOMs generated from container images are missing layers

[fabriziosestito]

Description

When scanning a SBOM generated from a container image, layers are missing from the scan results.
This happens because layers are explicitly removed if the scanned artifact is not of the type container image.
However, this removes the layers when a container image SBOM is generated from a container image.
Moreover, setting layers to an empty value is not needed as layers are not present when scanning artifacts that are not container images or SBOMs of container images anyway.

Related issues

• Related to discussion: Scanning an SBOM doesn't include container layer information, even if the SBOM specifies a type of container #6504

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