Remove FxpMathOps dialect and Quantizer tool.

Remove FxpMathOps dialect and Quantizer tool.

Summary:
* Removal of FxpMathOps was discussed on the mailing list.
* Will send a courtesy note about also removing the Quantizer (which had some dependencies on FxpMathOps).
* These were only ever used for experimental purposes and we know how to get them back from history as needed.
* There is a new proposal for more generalized quantization tooling, so moving these older experiments out of the way helps clean things up.

Subscribers: mgorny, mehdi_amini, rriddle, jpienaar, burmako, shauheen, antiagainst, nicolasvasilache, arpith-jacob, mgester, lucyrfox, liufengdb, Joonsoo, grosul1, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D77479