This PR removes an old workaround around non-implemented C++11 features
in the thread finalization.
This `ifdef` dates back to approximately 2015 as can be seen
[here](https://github.com/leanprover/lean3/blame/master/src/util/thread.cpp#L177),
the comments mention that it was originally implemented because not all
compilers at the time were able to support the C++11 `thread_local`
keyword. 10 years later this is hopefully the case and we can remove
this workaround.
There is an additional motivation for doing this,
`lean::initialize_thread` contains the following allocation:
```cpp
g_thread_finalizers_mgr = new thread_finalizers_manager;
```
this is supposed to be freed at some point but:
```cpp
// TODO(gabriel): race condition with thread finalizers
void delete_thread_finalizer_manager() {
// delete g_thread_finalizers_mgr;
// g_thread_finalizers_mgr = nullptr;
}
```
so `g_thread_finalizers_mgr` leaks upon repeated invocation of
`lean::initialize_thread`.
Note that Windows has already been using this alternative implementation
for a while so the alternative implementation has (hopefully) not rotten
away in the meantime.
50cfe354be