benchmark: disable only the ESLint rule needing it

In the spread-assign.js benchmark file, all ESLint rules are disabled
for a line where only no-unused-vars needs to be disabled. This change
makes it so that the remaining rules are still applied to the line.

PR-URL: nodejs#21133
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

benchmark/es/spread-assign.js

@@ -15,7 +15,7 @@ function main({ n, context, count, rest, method }) {
   for (let n = 0; n < count; n++)
     src[`p${n}`] = n;
 
-  let obj;  // eslint-disable-line
+  let obj;  // eslint-disable-line no-unused-vars
   let i;
 
   switch (method) {