Core: Remove IsHostThread

The core no longer cares which thread is the host thread.

Cleaning up Android's HostThreadLock is left for another PR, in part
because the HostThreadLock in NativeConfig.cpp still serves a purpose,
and in part to make any issues easier to bisect.

Source/Core/DolphinQt/Main.cpp

@@ -127,8 +127,6 @@ int main(int argc, char* argv[])
   }
 #endif
 
-  Core::DeclareAsHostThread();
-
 #ifdef __APPLE__
   // On macOS, a command line option matching the format "-psn_X_XXXXXX" is passed when
   // the application is launched for the first time. This is to set the "ProcessSerialNumber",

Source/Core/DolphinQt/Settings.cpp

@@ -17,6 +17,7 @@
 #include <QSize>
 #include <QStyle>
 #include <QStyleHints>
+#include <QThread>
 #include <QWidget>
 
 #include "AudioCommon/AudioCommon.h"
@@ -76,7 +77,7 @@ Settings::Settings()
   });
 
   m_hotplug_event_hook = g_controller_interface.RegisterDevicesChangedCallback([this] {
-    if (Core::IsHostThread())
+    if (qApp->thread() == QThread::currentThread())
     {
       emit DevicesChanged();
     }