Refactor performance bar cookie check

Refactors the method to make the intent clearer

Changelog: changed
MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/62675

app/controllers/concerns/with_performance_bar.rb

@@ -20,12 +20,11 @@ module WithPerformanceBar
   end
 
   def cookie_or_default_value
-    cookie_enabled = if cookies[:perf_bar_enabled].present?
-                       cookies[:perf_bar_enabled] == 'true'
-                     else
-                       cookies[:perf_bar_enabled] = 'true' if Rails.env.development?
-                     end
+    if cookies[:perf_bar_enabled].blank? && Rails.env.development?
+      cookies[:perf_bar_enabled] = 'true'
+    end
 
+    cookie_enabled = cookies[:perf_bar_enabled] == 'true'
     cookie_enabled && Gitlab::PerformanceBar.allowed_for_user?(current_user)
   end
 end