SkMipMap::Build supports 1xN and Nx1 inputs, so remove the special case from the test harness. Update a no-longer-correct comment.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1831983003

Review URL: https://codereview.chromium.org/1831983003
1 file changed