chore(tracemetrics): Remove attributes dropdown side panel flag (#112341)

Remove the `organizations:tracemetrics-attributes-dropdown-side-panel`
feature flag registration from the backend.

Refs LOGS-671

Made with [Cursor](https://cursor.com)

Author: nsdeschenes

src/sentry/features/temporary.py

@@ -486,8 +486,6 @@ def register_temporary_features(manager: FeatureManager) -> None:
     manager.add("organizations:ourlogs-table-expando", OrganizationFeature, FeatureHandlerStrategy.FLAGPOLE, api_expose=True)
     # Enable alerting on trace metrics
     manager.add("organizations:tracemetrics-alerts", OrganizationFeature, FeatureHandlerStrategy.FLAGPOLE, api_expose=True)
-    # Enable attributes dropdown side panel in trace metrics
-    manager.add("organizations:tracemetrics-attributes-dropdown-side-panel", OrganizationFeature, FeatureHandlerStrategy.FLAGPOLE, api_expose=True)
     # Enable trace metrics product (known internally as tracemetrics) in UI and backend
     manager.add("organizations:tracemetrics-enabled", OrganizationFeature, FeatureHandlerStrategy.FLAGPOLE, api_expose=True)
     # Enable equations in trace metrics explore