Remove dead malware marker code from PackageMetadata::SyncService

What does this MR do?

Removes dead code from PackageMetadata::SyncService: the full_sync_target_sequence resumable-first-sync marker lifecycle and the malware connector branches (MalwarePds, MalwareOffline).

Why

The marker lifecycle was added to SyncService in !244523 (merged), gated on sync_config.malware_advisories?. But SyncService only ever runs for the advisories, licenses, and cve_enrichment data types (via their dedicated workers) — none of which are malware or use the :pds storage type. Malware advisories are synced by PackageMetadata::MalwareAdvisorySyncService, which now owns the marker lifecycle (wired in !250919 (merged)). The marker code and the :pds / malware-offline connector branches in SyncService are therefore unreachable.

How

• #execute reverts to a plain checkpoint.update(sequence:, chunk:) per file.
• Removes #full_dataset_sync?, #checkpoint_position, and #finalize_full_sync.
• Removes the :pds -> MalwarePds connector branch and the malware branch of #offline_connector.
• Behaviour for advisories, licenses, and cve_enrichment is unchanged.

Testing

ee/spec/services/package_metadata/sync_service_spec.rb passes (50 examples, 0 failures); the specs that covered the removed malware/marker branches are dropped.

References

• Follow-up to #617851 (closed)
• Fix that moved the marker into the malware service: !250919 (merged)
• Marker originally added in !244523 (merged)