DiscIO/SConfig: Rename GetUniqueID to GetGameID

We call this "game ID" everywhere else, and it's not
actually completely unique.

Source/Core/VideoBackends/OGL/ProgramShaderCache.cpp

@@ -432,7 +432,7 @@ void ProgramShaderCache::Init()
 
       std::string cache_filename =
           StringFromFormat("%sogl-%s-shaders.cache", File::GetUserPath(D_SHADERCACHE_IDX).c_str(),
-                           SConfig::GetInstance().m_strUniqueID.c_str());
+                           SConfig::GetInstance().m_strGameID.c_str());
 
       ProgramShaderCacheInserter inserter;
       g_program_disk_cache.OpenAndRead(cache_filename, inserter);