lib: remove non used getter in lib/perf_hooks.js

Author: juanarbol

lib/perf_hooks.js

@@ -303,10 +303,6 @@ class PerformanceObserverEntryList {
     this[kCount]++;
   }
 
-  get length() {
-    return this[kCount];
-  }
-
   [kGetEntries](name, type) {
     const ret = [];
     const list = this[kEntries];