Remove libretro core - core will now be maintained by libretro

libretro have agreed to take over maintenance of the core.

Please see their fork at https://github.com/libretro/duckstation if you
wish to continue to use it.

src/frontend-common/opengl_host_display.cpp

@@ -9,7 +9,7 @@
 #include "imgui_impl_opengl3.h"
 #endif
 #include "postprocessing_shadergen.h"
-Log_SetChannel(LibretroOpenGLHostDisplay);
+Log_SetChannel(OpenGLHostDisplay);
 
 namespace FrontendCommon {