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/common/vulkan/shader_compiler.cpp

@@ -124,9 +124,7 @@ bool InitializeGlslang()
     return false;
   }
 
-#ifndef LIBRETRO
   std::atexit([]() { glslang::FinalizeProcess(); });
-#endif
 
   glslang_initialized = true;
   return true;