8305922: [aix,linux] Avoid comparing 'this' to nullptr

Reviewed-by: mdoerr, dholmes

src/hotspot/os/aix/osThread_aix.cpp

@@ -34,7 +34,6 @@
 #include "runtime/vmThread.hpp"
 
 void OSThread::pd_initialize() {
-  assert(this != nullptr, "check");
   _thread_id        = 0;
   _kernel_thread_id = 0;
   _siginfo = nullptr;

src/hotspot/os/linux/osThread_linux.cpp

@@ -30,7 +30,6 @@
 #include <signal.h>
 
 void OSThread::pd_initialize() {
-  assert(this != nullptr, "check");
   _thread_id        = 0;
   _pthread_id       = 0;
   _siginfo = nullptr;