LibCore+Userland: Don't auto-start new Core::Timers

This was unintuitive, and only useful in a few cases. In the majority,
users had to immediately call `stop()`, and several who did want the
timer started would call `start()` on it immediately anyway. Case in
point: There are only two places I had to add a manual `start()`.

Userland/DevTools/Profiler/main.cpp

@@ -329,6 +329,7 @@ static bool prompt_to_stop_profiling(pid_t pid, DeprecatedString const& process_
     auto update_timer = Core::Timer::construct(100, [&] {
         timer_label.set_text(DeprecatedString::formatted("{:.1} seconds", static_cast<float>(clock.elapsed()) / 1000.0f));
     });
+    update_timer->start();
 
     auto& stop_button = widget->add<GUI::Button>("Stop");
     stop_button.set_fixed_size(140, 22);