[Kernel] Do not retain handle in ExCreateThread

Closing the thread handle should delete the thread object
if it's finished. When this handle was retained, closing the handle
left a lingering handle reference and thus this XThread would leak

src/xenia/kernel/xboxkrnl/xboxkrnl_threading.cc

@@ -138,7 +138,6 @@ dword_result_t ExCreateThread(lpdword_t handle_ptr, dword_t stack_size,
       if (creation_flags & 0x80) {
         *handle_ptr = thread->guest_object();
       } else {
-        thread->RetainHandle();
         *handle_ptr = thread->handle();
       }
     }