nixos/binfmt: move systemd-binfmt.service to binfmt module

Also, remove the dangling systemd.services.systemd-binfmt.wants = [
"proc-sys-fs-binfmt_misc.mount" ]; in systemd.nix.

If boot.binfmt.registrations != {}, systemd will install
proc-sys-fs-binfmt_misc.automount, which will auto-mount
`/proc/sys/fs/binfmt_misc` as soon as systemd-binfmt tries to access it.

Fixes https://github.com/NixOS/nixpkgs/issues/87687
Fixes https://github.com/NixOS/nixops/issues/574