system_server: remove access sysfs_devices_system_cpu

CpuFrequency.java seems to be the only thing that depends on
/sys/devices/system/cpu in system_server. And according to
b/68988722#comment15, that dependency is not exercised.

Bug: 68988722
Test: walleye boots without denials to sysfs_devices_system_cpu
Change-Id: If777b716bf74188581327b7f5aa709f5d88aad2d