Replace duckstation-sdl with stub directing users to duckstation-qt

src/duckstation-sdl/CMakeLists.txt

@@ -1,17 +1,7 @@
 add_executable(duckstation-sdl
-  imgui_impl_sdl.cpp
-  imgui_impl_sdl.h
   main.cpp
-  sdl_host_interface.cpp
-  sdl_host_interface.h
-  sdl_key_names.h
-  sdl_util.cpp
-  sdl_util.h
 )
 
-target_include_directories(duckstation-sdl PRIVATE ${SDL2_INCLUDE_DIRS})
-target_link_libraries(duckstation-sdl PRIVATE core common imgui nativefiledialog glad frontend-common scmversion vulkan-loader ${SDL2_LIBRARIES})
-
 if(WIN32)
   target_sources(duckstation-sdl PRIVATE
     duckstation-sdl.manifest