[WebNN] Remove NHWC preferred layout (#25679)
Chromium has implemented constant folding for transpose which eliminates
the performance penalty introduced by additional transpose ops for
context with NHWC preferred layout.
Chromium CL:
https://chromium-review.googlesource.com/c/chromium/src/+/6774969
Now we can safety remove the NHWC optimization in WebNN EP.
