Remove unused Sbom::Occurrence.for_tracked_context scope

What does this MR do

Removes the Sbom::Occurrence.for_tracked_context scope (and its spec) added in !241041 (merged).

Why

!241041 (merged) added this scope to support reachability on tracked branches by filtering occurrences through sbom_occurrence_refs. A subsequent model review established that reachability is a characteristic of a specific finding (vulnerability_occurrence), not the deduplicated vulnerability, and the finding already carries the tracked context. The reachability consumers (!241042, !241043) therefore query by sbom_occurrences_vulnerabilities.vulnerability_occurrence_id (via the existing filter_by_vulnerability_occurrence_id scope) and do not use for_tracked_context.

That leaves for_tracked_context with no callers in the codebase, so this removes it to avoid leaving a misleading "this is how you scope reachability by context" entry point.

Confirmed no references remain:

$ git grep for_tracked_context
(no results)

Context

Part of the reachability-on-tracked-branches work under epic &21396. See the discussion on !241041 (merged) and the finding-keyed reworks in !241042 / !241043.