This reverts commit e1e6f294.

Reason for revert: This fix is incorrect, it ends up being a noop. Reverting this first before landing a different fix.

Original change's description:
> Make copyTexImage2D robust when source area is out of bounds
> 
> Whenever the source area of the texture being copied is out of
> bounds and robust resource init is enabled, it is necessary to
> clear the invalid portion of the source area to 0 in the copy
> operation.
> 
> Bug: angleproject:3930
> Change-Id: If1605b917097006c37e7d59c66babbc32d0ba6d8
> Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2378004
> Reviewed-by: Geoff Lang <geofflang@chromium.org>
> Reviewed-by: back sept 10 - Jamie Madill <jmadill@chromium.org>
> Commit-Queue: Alexis Hétu <sugoi@chromium.org>

TBR=sugoi@chromium.org,geofflang@chromium.org,jmadill@chromium.org,sugoi@google.com

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: angleproject:3930
Change-Id: I9e37a3f739eb7ce5d6d6ab5ca59d11ce9aae8483
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2388718Reviewed-by: Alexis Hétu <sugoi@chromium.org>
Commit-Queue: Alexis Hétu <sugoi@chromium.org>