Remove asserts on scissor size in Vulkan

These are asserts are firing from a recent change to our scissor code.
Since these asserts were added, the Vulkan spec has been updated to no
longer require the scissor is insides the bounds of the image, just that
x + width does not overflow.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2171283004

Review-Url: https://codereview.chromium.org/2171283004
1 file changed