Core: Remove SerialInterfaceManager::ChangeDevice

Now SerialInterfaceManager::UpdateDevices reads the configured SI
devices directly from Config instead.

The main reason why I wanted to do this is so that Android can change SI
devices while emulation is running. (Android didn't have the code for
calling ChangeDevice.) But when implementing the change, I noticed that
NetPlay and Movie were using ChangeDevice as a way of overriding the SI
devices configured by the user. Replacing this ended up making the change
larger than I first anticipated.

For Wii Remotes, there was no equivalent to ChangeDevice, so NetPlay and
Movie were using Config::SetCurrent to override the Wii Remote source
configured by the user. If we can use the config system to override Wii
Remote sources, why not do the same for SI devices? This commit makes
NetPlay and Movie set SI devices and Wii Remote sources in the NetPlay
and Movie config layers, as that is the conceptually appropriate place
to do it.

As far as I can tell, the old Movie code for overriding SI devices and
Wii Remote sources didn't actually work. This new code does. I didn't
investigate exactly why it didn't work, but maybe it's because loading
a movie happens before emulation actually starts.

Source/Core/DolphinQt/Config/GamecubeControllersWidget.cpp

@@ -15,7 +15,6 @@
 
 #include "Core/ConfigManager.h"
 #include "Core/Core.h"
-#include "Core/HW/SI/SI.h"
 #include "Core/HW/SI/SI_Device.h"
 #include "Core/NetPlayProto.h"
 #include "Core/System.h"
@@ -190,17 +189,11 @@ void GamecubeControllersWidget::SaveSettings()
   {
     Config::ConfigChangeCallbackGuard config_guard;
 
-    auto& system = Core::System::GetInstance();
     for (size_t i = 0; i < m_gc_groups.size(); ++i)
     {
       const SerialInterface::SIDevices si_device =
           FromGCMenuIndex(m_gc_controller_boxes[i]->currentIndex());
       Config::SetBaseOrCurrent(Config::GetInfoForSIDevice(static_cast<int>(i)), si_device);
-
-      if (Core::IsRunning(system))
-      {
-        system.GetSerialInterface().ChangeDevice(si_device, static_cast<s32>(i));
-      }
     }
   }