denoiser.hpp: simplify DPM++ (2S) Ancestral

[vmobilis]

DPMPP2S_A: https://github.com/leejet/stable-diffusion.cpp/blob/10c6501bd05a697e014f1bee3a84e5664290c489/denoiser.hpp#L686 Current formula:

if (sigma_down == 0) {
    d = (x - denoised) / sigmas[i];
    dt = sigma_down - sigmas[i];
    x += d * dt;
}

Since sigma_down == 0 => dt == -sigmas[i] => d * dt == (x - denoised) / sigmas[i] * (-sigmas[i]) == denoised - x => x = x + denoised - x => x = denoised

Proof:

Before	After

Images are almost same. And thank you for the program.