Remove unused sbom_components index in migration

What does this MR do and why?

This will remove the unused index_source_package_names_on_component_and_purl index in preparation for the removal of source_package_name from the sbom_components table.

Relates to Follow-up: remove source_package_name from sbom... (#435733 - closed) • Oscar Tovar • 17.3 • Needs attention

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