test: remove assert message

PR-URL: nodejs/node#15997
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: johenry

test/pummel/test-stream-pipe-multi.js

@@ -66,8 +66,8 @@ FakeStream.prototype.close = function() {
 
 // expect all streams to close properly.
 process.on('exit', function() {
-  assert.strictEqual(cnt, wclosed, 'writable streams closed');
-  assert.strictEqual(cnt, rclosed, 'readable streams closed');
+  assert.strictEqual(cnt, wclosed);
+  assert.strictEqual(cnt, rclosed);
 });
 
 for (let i = 0; i < chunkSize; i++) {