[mlir] Remove iterator_types() method from LinalgStructuredInterface.

`getIteratorTypesArray` should be used instead. It's a better substitute for all the current usages of the interface.

The current `ArrayAttr iterator_types()` has a few problems:
* It creates an assumption operation has iterators types as an attribute, but it's not always the case. Sometime iterator types can be inferred from other attribute, or they're just static.
* ArrayAttr is an obscure contained and required extracting values in the client code.
* Makes it hard to migrate iterator types from strings to enums ([RFC](https://discourse.llvm.org/t/rfc-enumattr-for-iterator-types-in-linalg/64535/9)).

Concrete ops, like `linalg.generic` will still have iterator types as an attribute if needed.

As a side effect, this change helps a bit with migration to prefixed accessors.

Differential Revision: https://reviews.llvm.org/D135765