[WebNN EP] Remove activation fusion (#20635)
WebNN spec has removed activation option for conv and
batchNormalization. We don't need additional activation fusion in WebNN
EP anymore.
[edit by fdwr] Note this is handled in the browser now, which knows more
about the backend platform version and can more safely make decisions
about which fusions are possible (e.g. for the DirectML backend, whether
softmax and gelu can fuse successfully with their base operator).
