lib: remove useless statement

There's no need to set 'false' to 'isAsync' because we've declared it
before.

PR-URL: #39983
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>

lib/internal/streams/from.js

@@ -25,7 +25,7 @@ function from(Readable, iterable, opts) {
     });
   }
 
-  let isAsync = false;
+  let isAsync;
   if (iterable && iterable[SymbolAsyncIterator]) {
     isAsync = true;
     iterator = iterable[SymbolAsyncIterator]();