[WebGPU] Delete some unnecessary methods from WebGPUExt.h
<rdar://problem/110201177>
Pull request: https://github.com/WebKit/WebKit/pull/14651
Committed 264893@main (5b9c48e708e7): <https://commits.webkit.org/264893@main> Reviewed commits have been landed. Closing PR #14651 and removing active labels.