…s_results scaffolding

These three methods were introduced to support the ASYNC_FINDING_IMPORT feature
(async dispatch of findings processing to Celery). That feature has since been
removed, leaving a three-layer indirection:

  process_scan -> determine_process_method -> sync_process_findings -> process_findings
                                                                 -> process_results

None of this routing logic has any effect anymore:
- sync_process_findings just delegates to process_findings
- determine_process_method just delegates to sync_process_findings
- process_results just returns self.new_items / self.reactivated_items / etc.

Collapse the call chain so process_scan calls process_findings directly and
process_findings returns the finding lists directly. This also fixes the
indirection that was the root cause of DefectDojo#14309's performance test failures.