um: fix unused variable warning

um: fix unused variable warning

The code was updated to access the PID of the userspace stub process in
a different way, making the local cpu variable obsolete. Remove it.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202506050008.AwXLNxQX-lkp@intel.com/
Fixes: 406d17c6c370 ("um: Implement kernel side of SECCOMP based process handling")
Signed-off-by: Benjamin Berg <benjamin.berg@intel.com>
Link: https://patch.msgid.link/20250605050325.1077208-1-benjamin@sipsolutions.net
Signed-off-by: Johannes Berg <johannes.berg@intel.com>