[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);}
```

llvm/lib/Support/Unix/ThreadLocal.inc

@@ -17,8 +17,6 @@
 
 #include "llvm/Config/config.h"
 
-#if defined(HAVE_PTHREAD_H) && defined(HAVE_PTHREAD_GETSPECIFIC)
-
 #include <cassert>
 #include <pthread.h>
 #include <stdlib.h>
@@ -58,13 +56,3 @@ void ThreadLocalImpl::removeInstance() {
 }
 
 }
-#else
-namespace llvm {
-using namespace sys;
-ThreadLocalImpl::ThreadLocalImpl() : data() { }
-ThreadLocalImpl::~ThreadLocalImpl() { }
-void ThreadLocalImpl::setInstance(const void* d) { data = const_cast<void*>(d);}
-void *ThreadLocalImpl::getInstance() { return data; }
-void ThreadLocalImpl::removeInstance() { setInstance(0); }
-}
-#endif