Revert "debugging rxjs"

This reverts commit ebfa0e6.

Author: zarend

src/cdk/tree/tree.ts

@@ -399,16 +399,12 @@ export class CdkTree<T, K = T>
         this._nodeType,
         // NB: the data is unused below, however we add it here to essentially
         // trigger data rendering when expansion changes occur.
-        //
-        // TODO: remove debug code and comments
-        // Temporarily commenting this out to debug a test failure
-        //
-        // expansionModel.changed.pipe(
-        //   startWith(null),
-        //   tap(expansionChanges => {
-        //     this._emitExpansionChanges(expansionChanges);
-        //   }),
-        // ),
+        expansionModel.changed.pipe(
+          startWith(null),
+          tap(expansionChanges => {
+            this._emitExpansionChanges(expansionChanges);
+          }),
+        ),
       ])
         .pipe(
           switchMap(([data, nodeType]) => {