Remove dead code from update_client::Component.

components/update_client/component.cc

Index: components/update_client/component.cc
diff --git a/components/update_client/component.cc b/components/update_client/component.cc
index e83e325ba51eb1b68e3c71fd0bd90ae03d52b913..350f3d093a3df62b1f0205f7888b9bbe51fd9d4f 100644
--- a/components/update_client/component.cc
+++ b/components/update_client/component.cc
@@ -615,8 +615,7 @@ void Component::StateUpdatingDiff::InstallComplete(int error_category,
 }
 
 Component::StateUpdating::StateUpdating(Component* component)
-    : State(component, ComponentState::kUpdating),
-      main_task_runner_(base::ThreadTaskRunnerHandle::Get()) {}
+    : State(component, ComponentState::kUpdating) {}
 
 Component::StateUpdating::~StateUpdating() {
   DCHECK(thread_checker_.CalledOnValidThread());