um: Remove duplicate arch.h header

./arch/um/kernel/trap.c: arch.h is included more than once.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Closes: https://bugzilla.openanolis.cn/show_bug.cgi?id=19877


Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Link: https://patch.msgid.link/20250331083150.72598-1-jiapeng.chong@linux.alibaba.com


Signed-off-by: Johannes Berg <johannes.berg@intel.com>