Remove the constant tensor support

[mingmingtasd]

The constant tensor feature allows tensors to be created with initial data and used as graph constants. However, because no backends currently support constant tensors, we should remove this feature to simplify the code.
We should remove the API createconstanttensor and readonly attribute boolean constant.
@huningxin @fdwr @RafaelCintron @reillyeon

[bbernhar]

The original motivation for constant tensors (issue #760) was to enable reusable immutable weights.

Has that optimization path been abandoned, or is there another reason the original rationale no longer applies? It would be helpful to document that in the issue.

[reillyeon]

Given the only backend which supported it was removed from the Chromium prototype I think the working group needs to redo the analysis of whether this approach is implementable or if there is an alternative mechanism to achieve the same optimization.