[mlir] Remove obsolete "Quantization" section from the rationale.

[stellaraccident]

Summary

• It reads as more of a TODO for the future and has been long obsoleted by later work.
• One of the authors of the referenced paper called this out as "weird stuff from two years ago" when reviewing the more recent TOSA RFC.

mlir/docs/Rationale/Rationale.md

 semantic barriers, making it possible to unknowingly break region semantics.
 This is expected to be caught in the verifier after the transformation.
 
 At the same time, one may choose to pass certain or all values as region
 arguments to explicitly break the use-def chains in the current proposal. This
 can be combined with an attribute-imposed semantic requirement disallowing the
 body of the region to refer to any value from outside it.
 
-### Quantized integer operations
-
-We haven't designed integer quantized operations in MLIR, but experience from
-TensorFlow suggests that it is better to put information about the quantization
-range/scale into the type itself, rather than have a single type like "qint8"
-and put these on attributes of the operation.
-
-There are a few ways to do this with MLIR, including at least:
-
-*   We could do the same thing TensorFlow does - and we will _have_ to support
-    that model to some extent for compatibility.
-*   We can encode the fp range of quantized integers directly into the types
-    when they are constants. The best practice on this seems to be to encode the
-    zero point as well as a scale factor. This ensures that 0.0 is always
-    exactly representable, e.g. `qi8<-1.42, 31.23x>`.
-*   We could theoretically encode dynamically determined ranges into the types
-    using something like `qi8<?,?>` with the bounds being determined through the
-    SSA dataflow graph dynamically - similar to how dynamic shapes are handled.
-
-We will definitely need to do #1 for compatibility, we probably want to do #2,
-and we should investigate #3 over time. That said, our short term plan is to get
-more implementation experience with the rest of the system first, then come back
-to re-examine the representation for quantized arithmetic when we have that
-experience. When we do, we should chat with benoitjacob@ and
-[read the paper](https://arxiv.org/abs/1712.05877).
-
 ### Dialect type extensions
 
 This section describes the design decisions that shaped the dialect extensible
 type system present in MLIR.
 
 #### Interactions between dialects
 
 There are two different interactions between dialects that are important to