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/Applications/SoundPlayer/PlaybackManager.cpp

@@ -16,7 +16,6 @@ PlaybackManager::PlaybackManager(NonnullRefPtr<Audio::ConnectionToServer> connec
             return;
         next_buffer();
     });
-    m_timer->stop();
     m_device_sample_rate = connection->get_sample_rate();
 }