[MLIR][AsyncToLLVM] Remove typed pointer support (#70731)

This commit removes the support for lowering Async to LLVM dialect with
typed pointers. Typed pointers have been deprecated for a while now and
it's planned to soon remove them from the LLVM dialect.

Related PSA:
https://discourse.llvm.org/t/psa-removal-of-typed-pointers-from-the-llvm-dialect/74502