[Support] Remove unused/uncompilable !HAVE_PTHREAD_GETSPECIFIC code path

lib/Support/ThreadLocal.cpp has been uncompilable since rL158346 (2012-06) when
`data` became a char array. The error looks like
```
...llvm/lib/Support/Unix/ThreadLocal.inc:66:57: error: array type 'char[8]' is not assignable
void ThreadLocalImpl::setInstance(const void* d) { data = const_cast<void*>(d);}
```