Update cropped sprites for US/UM

Spiky-Eared Pichu, Cosplay Pikachu, and Unknown-type Arceus have no
sprites in the current generation (b/c they don't exist) so their old
cropped sprites have been left untouched.

    for spr in main-sprites/ultra-sun-ultra-moon/*.png(n); do convert "$spr" -trim cropped-usum/"${spr##*/}"; done
    optipng cropped-usum/*.png
    advdef -z3 cropped-usum/*.png
    mv cropped-usum/*.png cropped

Fixes https://github.com/veekun/spline-pokedex/issues/127