nixos/hidepid: drop the module as the hidepid mount option is broken

This has been in an unusable state since the switch to cgroups-v2.
See https://github.com/NixOS/nixpkgs/issues/73800 for details.

nixos/tests/hardened.nix

@@ -65,17 +65,6 @@ import ./make-test-python.nix ({ pkgs, latestKernel ? false, ... } : {
           machine.succeed("grep -Fq wireguard /proc/modules")
 
 
-      # Test hidepid
-      with subtest("hidepid=2 option is applied and works"):
-          # Linux >= 5.8 shows "invisible"
-          machine.succeed(
-              "grep -Fq hidepid=2 /proc/mounts || grep -Fq hidepid=invisible /proc/mounts"
-          )
-          # cannot use pgrep -u here, it segfaults when access to process info is denied
-          machine.succeed("[ `su - sybil -c 'ps --no-headers --user root | wc -l'` = 0 ]")
-          machine.succeed("[ `su - alice -c 'ps --no-headers --user root | wc -l'` != 0 ]")
-
-
       # Test kernel module hardening
       with subtest("No more kernel modules can be loaded"):
           # note: this better a be module we normally wouldn't load ...