[MLIR][NFC] Remove unnecessary cast.

I was reading this post:
https://www.fluentcpp.com/2017/05/19/crtp-helper/

And I noticed that most likely this cast is not needed.
Unless it is needed by some compiler versions.
I tested it with:
cmake --build . --target check-mlir

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D121555