[Sync] Clean up SyncSendInterestedDataTypes feature toggle

The feature was enabled by default a while ago.

There is no behavior change. This CL also cleans up nullness checks for
SyncInvalidationsService since it should always exist now.

Bug: 1404927
Change-Id: I81700f6c67aa0d581bf78d3e2f7d1104786410db
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4136122
Commit-Queue: Rushan Suleymanov <[email protected]>
Reviewed-by: Mihai Sardarescu <[email protected]>
Reviewed-by: Marc Treib <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1089192}
diff --git a/components/sync/driver/sync_api_component_factory.h b/components/sync/driver/sync_api_component_factory.h
index 4794eab..4baadaa 100644
--- a/components/sync/driver/sync_api_component_factory.h
+++ b/components/sync/driver/sync_api_component_factory.h
@@ -40,12 +40,12 @@
   // and |sync_invalidation_service| are different invalidations. SyncEngine
   // handles incoming invalidations from both of them (if provided).
   // |sync_invalidation_service| is a new sync-specific invalidations service
-  // and it may be nullptr if it is disabled or not supported. In future, there
-  // will leave only one invalidation service.
+  // and must not be null. In future, there will be only one invalidation
+  // service.
   virtual std::unique_ptr<SyncEngine> CreateSyncEngine(
       const std::string& name,
       invalidation::InvalidationService* invalidator,
-      syncer::SyncInvalidationsService* sync_invalidation_service) = 0;
+      SyncInvalidationsService* sync_invalidation_service) = 0;
 
   // Clears all local transport data. Upon calling this, the deletion is
   // guaranteed to finish before a new engine returned by |CreateSyncEngine()|