x86/process: Delete useless check for dead process with LDT

At release_thread(), ->mm is NULL; and it is fine for the former mm to
still have an LDT. Delete this check in process_64.c, similar to
commit 2684927c ("[PATCH] x86: Deprecate useless bug"), which did the
same in process_32.c.
Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20190712224152.13129-1-jannh@google.com

arch/x86/kernel/process_64.c

@@ -143,17 +143,7 @@ void __show_regs(struct pt_regs *regs, enum show_regs_mode mode)
 
 void release_thread(struct task_struct *dead_task)
 {
-	if (dead_task->mm) {
-#ifdef CONFIG_MODIFY_LDT_SYSCALL
-		if (dead_task->mm->context.ldt) {
-			pr_warn("WARNING: dead process %s still has LDT? <%p/%d>\n",
-				dead_task->comm,
-				dead_task->mm->context.ldt->entries,
-				dead_task->mm->context.ldt->nr_entries);
-			BUG();
-		}
-#endif
-	}
+	WARN_ON(dead_task->mm);
 }
 
 enum which_selector {