Simplify StoreWriter#writeSelectionSetToStore using store.get.

Follow-up to #5919 and 9fbcda9.

Now that the policies.rootTypenamesById check happens in store.get, we
don't have to rely on the getFieldValue wrapper to ensure __typename can
always be determined.

src/cache/inmemory/writeToStore.ts

@@ -127,18 +127,17 @@ export class StoreWriter {
     if (sets.indexOf(selectionSet) >= 0) return store;
     sets.push(selectionSet);
 
-    const entityRef = makeReference(dataId);
     const typename =
       // If the result has a __typename, trust that.
       getTypenameFromResult(result, selectionSet, context.fragmentMap) ||
       // If the entity identified by dataId has a __typename in the store,
       // fall back to that.
-      store.getFieldValue<string>(entityRef, "__typename");
+      store.get(dataId, "__typename") as string;
 
     store.merge(
       dataId,
       policies.applyMerges(
-        entityRef,
+        makeReference(dataId),
         this.processSelectionSet({
           result,
           selectionSet,