seccomp: drop getrandom() from @system-service
It's included in @default now, since
14f4b1b568907350d023d1429c1aa4aaa8925f22, and since @system-service
pulls that in we can drop it from @system-service.
Follow-up for #20191
(cherry picked from commit 67347f37407489a68e12da8f75b78ae1d1168de9)