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.cpp

@@ -11,12 +11,10 @@
 #include <utility>
 Log_SetChannel(GameSettings);
 
-#ifndef LIBRETRO
 #ifdef WIN32
 #include "common/windows_headers.h"
 #endif
 #include "SimpleIni.h"
-#endif
 
 namespace GameSettings {
 
@@ -183,8 +181,6 @@ bool Entry::SaveToStream(ByteStream* stream) const
          WriteStringToStream(stream, memory_card_2_shared_path) && WriteStringToStream(stream, input_profile_name);
 }
 
-#ifndef LIBRETRO
-
 static void ParseIniSection(Entry* entry, const char* section, const CSimpleIniA& ini)
 {
   for (u32 trait = 0; trait < static_cast<u32>(Trait::Count); trait++)
@@ -897,8 +893,6 @@ void Entry::SetValueForKey(const std::string_view& key, const std::optional<std:
   SetEntryValueForKey(*this, key, value);
 }
 
-#endif
-
 void Entry::ApplySettings(bool display_osd_messages) const
 {
   constexpr float osd_duration = 10.0f;