Revert spdlog stdout_sink_mt workaround — fixed upstream #305
Description
The Windows-specific workaround that uses stdout_sink_mt instead of stdout_color_sink_mt to avoid WriteConsole failures under stdout redirection can be reverted.
The underlying issue (gabime/spdlog#1147) was fixed upstream in spdlog via commits c138685 and 72f3d52 (closed July 2019). Recent versions of spdlog handle redirected stdout correctly with the color sink.
The #ifdef _WIN32 sink selection can be removed in favor of using stdout_color_sink_mt on all platforms.