Remove useless CHECK in BaseRenderingContext2D::getTextureFormat()

getTextureFormat was originally using V8GPUTextureFormat::Create, which
parsed a string and returned std::nullopt on parsing failures.
https://crrev.com/c/5919677 however removed string<=>enum conversions.
The code can no longer fail.

Bug: 336288005
Change-Id: Id203acf45f05a7fb121b4d6524a0d3e957133377
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6235186
Reviewed-by: Vasiliy Telezhnikov <vasilyt@chromium.org>
Commit-Queue: Jean-Philippe Gravel <jpgravel@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1416312}