nios2: Remove unused prepare_to_copy()

prepare_to_copy() was removed from all architectures supported at that
time in commit 55ccf3fe ("fork: move the real prepare_to_copy()
users to arch_dup_task_struct()"). Remove it from nios2 as well.
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Acked-by: Ley Foon Tan <lftan@altera.com>

arch/nios2/include/asm/processor.h

@@ -85,9 +85,6 @@ static inline void exit_thread(void)
 
 extern unsigned long get_wchan(struct task_struct *p);
 
-/* Prepare to copy thread state - unlazy all lazy status */
-#define prepare_to_copy(tsk)	do { } while (0)
-
 #define task_pt_regs(p) \
 	((struct pt_regs *)(THREAD_SIZE + task_stack_page(p)) - 1)