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 arm64 as well.
Signed-off-by: NTobias Klauser <tklauser@distanz.ch>
Signed-off-by: NWill Deacon <will.deacon@arm.com>