Remove SkImageMinRowBytes

Remove SkImageMinRowBytes

As stated in the comments in crrev.com/1379193002, this method name is
misleading. It returns a larger number than the minimum rowbytes - it
increases to the next four-byte alignment. This has the effect that the
one place that calls it does not support 565 (which is not already
four-byte aligned), but does not serve any other purpose. Remove it and
the only call-site.

BUG=skia:4396
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1528383004

Review URL: https://codereview.chromium.org/1528383004