arm64: remove unneeded BUILD_BUG_ON assertion

Since commit c02433dd ("arm64: split thread_info from task stack"),
CONFIG_THREAD_INFO_IN_TASK is enabled unconditionally for arm64. So
remove this always-true assertion from arch_dup_task_struct.

Signed-off-by: Dawei Li <dawei.li@shingroup.cn>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/r/20240202040211.3118918-1-dawei.li@shingroup.cn


Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>