Remove ci_atomic_processing and ci_composite_status feature flags

In #197930 (closed) we rolled out ci_atomic_processing and ci_composite_status feature flags.

This features are a replacement for the Ci::PipelineProcessing::LegacyProcessingService.

When removing the feature flag ci_atomic_processing we should also remove:

• Ci::PipelineProcessing::LegacyProcessingService

Steps:

• Remove FF ci_atomic_processing and Ci::PipelineProcessing::LegacyProcessingService (!37339 (merged))
• Remove update_legacy_status and latest_builds_status (!39211 (merged))
  • suggestion: !37339 (comment 391423105)
• Remove FF ci_composite_status (!39498 (merged))
• Clean FFs