arc: 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 arc as well.
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>

arch/arc/include/asm/processor.h

@@ -56,9 +56,6 @@ unsigned long thread_saved_pc(struct task_struct *t);
 /* Free all resources held by a thread */
 #define release_thread(thread) do { } while (0)
 
-/* Prepare to copy thread state - unlazy all lazy status */
-#define prepare_to_copy(tsk)    do { } while (0)
-
 /*
  * A lot of busy-wait loops in SMP are based off of non-volatile data otherwise
  * get optimised away by gcc