[tools][system-analyzer] Delete Stats Panel duplicated getter/setter

This CL deletes the duplicated getters and setters
for timeline in Stats panel.

Change-Id: I3a08cfc84e6d099084cb5eeb0e25526a76a891a0
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2301930Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Zeynep Cankara <zcankara@google.com>
Cr-Commit-Position: refs/heads/master@{#68895}

tools/system-analyzer/stats-panel.mjs

@@ -40,14 +40,6 @@ defineCustomElement('stats-panel', (templateText) =>
     return this.timeline_;
   }
 
-  set timeline(value){
-    this.timeline_ = value;
-  }
-
-  get timeline(){
-    return this.timeline_;
-  }
-
   update() {
     this.removeAllChildren(this.stats);
     this.updateGeneralStats();