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

arch/arm64/kernel/process.c

@@ -290,9 +290,6 @@ int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src)
 		fpsimd_preserve_current_state();
 	*dst = *src;
 
-	/* We rely on the above assignment to initialize dst's thread_flags: */
-	BUILD_BUG_ON(!IS_ENABLED(CONFIG_THREAD_INFO_IN_TASK));
-
 	/*
 	 * Detach src's sve_state (if any) from dst so that it does not
 	 * get erroneously used or freed prematurely.  dst's copies