Revert "btrfs-progs: fix musl build"
Revert "btrfs-progs: fix musl build"

This reverts commit a3d54dece70888e851a00a28a1895ea6b1928fdf.

We now support systemd on Musl, so we don't need to disable
btrfs-progs' support for it by default on Musl.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs