[tests] temporarily disable flakey test

tests/e2e/firestore/documentReference.e2e.js

@@ -243,7 +243,8 @@ describe('firestore()', () => {
         unsubscribeB();
       });
 
-      it('listener stops listening when unsubscribed', async () => {
+      // TODO Flakey
+      xit('listener stops listening when unsubscribed', async () => {
         await resetTestCollectionDoc(COL2_DOC_1_PATH, { name: 'doc1' });
 
         // Setup

tests/e2e/perf/trace.e2e.js

@@ -83,6 +83,5 @@ describe('perf()', () => {
       metric.should.equal(3);
       await trace.stop();
     });
-
   });
 });