benchmark: simplify http benchmarker regular expression

A non-capturing group inside a capturing group has no effect. Simplify
the regular expression.

PR-URL: nodejs#38206
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ricky Zhou <0x19951125@gmail.com>

benchmark/_http-benchmarkers.js

@@ -173,7 +173,7 @@ class H2LoadBenchmarker {
   }
 
   processResults(output) {
-    const rex = /(\d+(?:\.\d+)) req\/s/;
+    const rex = /(\d+\.\d+) req\/s/;
     return rex.exec(output)[1];
   }
 }