webnn: Remove webnn::Operand

All the validation code now uses OperandDescriptor

Bug: 325598628
Cq-Include-Trybots: luci.chromium.try:win11-blink-rel
Change-Id: Ibb4782980fabebb66e445a5ff41797203b2f041b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5637756


Commit-Queue: Austin Sullivan <asully@chromium.org>
Reviewed-by: ningxin hu <ningxin.hu@intel.com>
Cr-Commit-Position: refs/heads/main@{#1316545}