nixos/filesystems: succeed mount-pstore.service without backend

[ghost]

Motivation for this change

Unbreaks the simple test from installer.nix, which is currently blocking the nixos-unstable channel.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• 21.11 Release Notes (or backporting 21.05 Relase notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

[ajs124]

Why do we even need this mount unit in the first place? systemd should just mount this automatically: systemd/systemd@c06bf41

[ghost]

In NixOS systemd isn't responsible for mounting any of the special filesystems. I am not sure why that is. Probably just for legacy reasons.

[ghost]

The blocking test now suceeds.

[happysalada]

I confirm I've been getting the following error in the last couple of days

89ji0y3y98aq4kppbfi01-mount-pstore.sh[3284118]: Persistent Storage backend was not registered in time.
: mount-pstore.service: Main process exited, code=exited, status=1/FAILURE
: mount-pstore.service: Failed with result 'exit-code'.
: Failed to start mount-pstore.service.

and this PR removes the ERROR.

[ajs124]

Let's merge this, to unblock the channel and figure out why systemd doesn't mount those things automatically later.