[mlir] Delete most of the ops from the quant dialect.

* https://discourse.llvm.org/t/rfc-removing-the-quant-dialect/3643/8
* Removes most ops. Leaves casts given final comment (can remove more in a followup).
* There are a few uses in Tosa keeping some of the utilities alive. In a followup, I will probably elect to just move simplified versions of them into Tosa itself vs having this quasi-library dependency.

Differential Revision: https://reviews.llvm.org/D120204