buffer: remove unnecessary assignment in fromString

The assignment to the encoding variable has no effect.

Refs: nodejs#29217
PR-URL: nodejs#50199
Reviewed-By: Yagiz Nizipli <yagiz@nizipli.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

Author: tniessen

lib/buffer.js

@@ -475,7 +475,6 @@ function fromString(string, encoding) {
     if (string.length === 0)
       return new FastBuffer();
     ops = encodingOps.utf8;
-    encoding = undefined;
   } else {
     ops = getEncodingOps(encoding);
     if (ops === undefined)