webnn: Remove some activations not usable with recurrent ops
clamp() and softmax() may no longer be created as MLActivations
More details on the spec PR:
webmachinelearning/webnn#703
Bug: 341518634
Change-Id: I7dda713c8be5454690f21c66665d90be274513f7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5598089
Commit-Queue: Austin Sullivan asully@chromium.org
Reviewed-by: Alex Gough ajgo@chromium.org
Reviewed-by: ningxin hu ningxin.hu@intel.com
Cr-Commit-Position: refs/heads/main@{#1313205}