Remove the support of `null` value in `newShape` parameter of `reshape` build method

[huningxin]

WebNN reshape op build method currently supports a special null value in newShape array that instructs the implementation to compute the size of the dimension with the value null so that the total size remains constant.

In PR review #367 (comment), @fwdr proposed WebNN should only support the concrete values in newShape because this can be easily handled by framework through shape inference. Thanks Dwyane!

Though also, is this some behavior that could be easily resolved by the frameworks calling into WebNN, since presumably they already know the shapes along the way (shape inference) and could instead pass concrete values to WebNN?

I think this would also help simplify the implementation.

/cc @wacky6

[wacky6]

SGTM from implementer perspective :)

[fdwr]

Repeating @wacky6's statement, it sounds good from an implementor perspective 😃. Even if this special policy would be convenient for end users, we expect low level WebNN to be called by ML frameworks/libraries, which can resolve the shape into something concrete.