[minor] Fix misleading comment

As of commit nodejs/node@0e89b64d66 the
`zlib.flush()` callback can be called if `zlib.close()` is called
while data is being processed.

lib/permessage-deflate.js

@@ -424,10 +424,7 @@ class PerMessageDeflate {
     this._deflate.flush(zlib.Z_SYNC_FLUSH, () => {
       if (!this._deflate) {
         //
-        // This `if` statement is only needed for Node.js < 10.0.0 because as of
-        // commit https://github.com/nodejs/node/commit/5e3f5164, the flush
-        // callback is no longer called if the deflate stream is closed while
-        // data is being processed.
+        // The deflate stream was closed while data was being processed.
         //
         return;
       }