[llvm] [BOLT][DWARF] Remove deprecated opt (PR #99575)

[Alexander Yermolovich via llvm-commits]

ayermolo wrote:

> > What about the rest of the code that this option depends on?
> 
> Sorry, what code are you referring to? It doesn't depend on any other code, it's only used/referenced in DWARFRewriter.cpp.

What started this discussion:

```
 // Update unit debug info in parallel
    ThreadPoolInterface &ThreadPool = ParallelUtilities::getThreadPool();
    for (std::unique_ptr<DWARFUnit> &CU : BC.DwCtx->compile_units())
      ThreadPool.async(processUnitDIE, CU.get(), &DIEBlder);
    ThreadPool.wait();
```

https://github.com/llvm/llvm-project/pull/99575