Remove assignment to undeclared variable in perf.js.

Former-commit-id: 6c8c6bd2db9a3ab137f6a705e2dabb2c07ccdc34

perf/perf.js

@@ -1284,7 +1284,7 @@
         'teardown': 'function omit(){}'
       })
       .add(otherName, {
-        'fn': 'result = _.omit(wordToNumber, words)',
+        'fn': '_.omit(wordToNumber, words)',
         'teardown': 'function omit(){}'
       })
   );