Tempo: remove backend migration feature toggle (#116054)

* remove unused frontend code

* remove feature toggle definition

* fix tests

packages/grafana-data/src/types/featureToggles.gen.ts

@@ -1137,11 +1137,6 @@ export interface FeatureToggles {
   */
   pluginContainers?: boolean;
   /**
-  * Run search queries through the tempo backend
-  * @default false
-  */
-  tempoSearchBackendMigration?: boolean;
-  /**
   * Prioritize loading plugins from the CDN before other sources
   * @default false
   */