fix: remove references to non-existent tracked_objects.h in platform_thread_freebsd.cc (#3223)

The tracked_objects.h header and ThreadData::InitializeThreadContext()
were part of Chromium's base library profiling subsystem, which was
never ported to brpc. The Linux (platform_thread_linux.cc) and macOS
(platform_thread_mac.mm) equivalents already had these references
removed. This causes a compile error on FreeBSD:

  fatal error: 'butil/tracked_objects.h' file not found

Author: tuaris

src/butil/threading/platform_thread_freebsd.cc

@@ -13,7 +13,6 @@
 #include "butil/safe_strerror_posix.h"
 #include "butil/threading/thread_id_name_manager.h"
 #include "butil/threading/thread_restrictions.h"
-#include "butil/tracked_objects.h"
 
 #if !defined(OS_NACL)
 #include <sys/resource.h>
@@ -46,7 +45,6 @@ int ThreadNiceValue(ThreadPriority priority) {
 // static
 void PlatformThread::SetName(const char* name) {
   ThreadIdNameManager::GetInstance()->SetName(CurrentId(), name);
-  tracked_objects::ThreadData::InitializeThreadContext(name);
 
   SetNameSimple(name);
 }