nios2: Remove unused prepare_to_copy()

prepare_to_copy() was removed from all architectures supported at that
time in commit 55ccf3fe3f9a ("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>