Remove the standalone frontends

The libretro core is the frontend, and the headless tools are the test and
debugging hosts. The Windows, UWP, Android, iOS, macOS and SDL apps, the UI
framework and imgui go, along with what only they used: the Win32 debugger
hooks (g_frameMutex, the shutdown lock), JNI thread attachment, the Android
text drawer, RAIntegration, Discord, recent files, game installs, tilt input,
the audio resamplers, the control mapper, memstick moving and the remote ISO
and upload server modes. The web server keeps the debugger. The MSVC, Gradle
and ndk-build projects go too.

Common/Thread/ThreadUtil.cpp

@@ -8,8 +8,6 @@
 
 #elif defined(__ANDROID__)
 
-#include "android/jni/app-android.h"
-
 #define TLS_SUPPORTED
 
 #endif
@@ -24,31 +22,6 @@
 #include "Common/Thread/ThreadUtil.h"
 #include "Common/Data/Encoding/Utf8.h"
 
-AttachDetachFunc g_attach;
-AttachDetachFunc g_detach;
-
-void AttachThreadToJNI() {
-	if (g_attach) {
-		g_attach();
-	} else {
-#if PPSSPP_PLATFORM(ANDROID)
-		// Not relevant on other platforms.
-		ERROR_LOG(Log::System, "Couldn't attach thread - g_attach not set");
-#endif
-	}
-}
-
-void DetachThreadFromJNI() {
-	if (g_detach) {
-		g_detach();
-	}
-}
-
-void RegisterAttachDetach(AttachDetachFunc attach, AttachDetachFunc detach) {
-	g_attach = attach;
-	g_detach = detach;
-}
-
 #if (PPSSPP_PLATFORM(ANDROID) || PPSSPP_PLATFORM(LINUX)) && !defined(_GNU_SOURCE)
 #define _GNU_SOURCE
 #endif