[GPU] Remove usage of GLImage::GetSize() in CmdDecoderImpl

We changed the validating decoder to always get the size from the source
texture with a Finch killswitch in 112. There have been no problems with
the rollout and the old NaCL GPU codepaths are now definitively gone, so
we can rip out the unused code here (note: I was waiting until the old
NaCL GPU codepaths were definitively gone just to avoid any potential
interactions between those codepaths being re-enabled and somehow being
dependent on this behavior). This CL does so.

Bug: 1412075
Change-Id: Ic937e99a2460d6298f46f189d1240e78da62250a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4551253


Reviewed-by: Vasiliy Telezhnikov <vasilyt@chromium.org>
Commit-Queue: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1148939}