chore(aci milestone 3): remove dual write feature flag registration (#101979)

Now that we've removed the options configuration from flagpole, we can
delete this feature flag from the codebase!

Author: mifu67

src/sentry/features/temporary.py

@@ -515,8 +515,6 @@ def register_temporary_features(manager: FeatureManager) -> None:
     manager.add("organizations:workflow-engine-trigger-actions", OrganizationFeature, FeatureHandlerStrategy.FLAGPOLE, api_expose=False)
     # Enable logs to debug metric alert dual processing
     manager.add("organizations:workflow-engine-metric-alert-dual-processing-logs", OrganizationFeature, FeatureHandlerStrategy.FLAGPOLE, api_expose=False)
-    # Enable dual writing for metric alert issues (see: alerts create issues)
-    manager.add("organizations:workflow-engine-metric-alert-dual-write", OrganizationFeature, FeatureHandlerStrategy.FLAGPOLE, api_expose=False, default=True)
     # Enable Processing for Metric Alerts in the workflow_engine
     manager.add("organizations:workflow-engine-metric-alert-processing", OrganizationFeature, FeatureHandlerStrategy.FLAGPOLE, api_expose=False)
     # Enable Creation of Metric Alerts that use the `group_by` field in the workflow_engine