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/game_settings.h

@@ -93,8 +93,6 @@ struct Entry
   void SetValueForKey(const std::string_view& key, const std::optional<std::string>& value);
 };
 
-#ifndef LIBRETRO
-
 class Database
 {
 public:
@@ -110,6 +108,4 @@ private:
   std::unordered_map<std::string, Entry> m_entries;
 };
 
-#endif
-
 }; // namespace GameSettings