[MLIR][NFC] Retire let constructor for Async (#137461)
let constructor is legacy (do not use in tree!) since the tableGen
backend emits most of the glue logic to build a pass.
Note: The following constructor has been retired:
```cpp
std::unique_ptr<Pass> createAsyncParallelForPass(bool asyncDispatch,
int32_t numWorkerThreads,
int32_t minTaskSize);
```
To update your codebase, replace it with the new options-based API:
```cpp
AsyncParallelForPassOptions options{/*asyncDispatch=*/, /*numWorkerThreads=*/, /*minTaskSize=*/};
createAsyncParallelForPass(options);
```
