doc: don't mention obsolete ssh-substituter-hosts
doc: don't mention obsolete ssh-substituter-hosts

#1840
ehmry/nix,shlevy/nix,ehmry/nix,vcunat/nix,layus/nix,peti/nix,dtzWill/nix,peti/nix,dezgeg/nix,NixOS/nix,ehmry/nix,NixOS/nix,dtzWill/nix,rycee/nix,dezgeg/nix,layus/nix,NixOS/nix,pikajude/nix,ehmry/nix,zimbatm/nix,vcunat/nix,dezgeg/nix,rycee/nix,layus/nix,vcunat/nix,pikajude/nix,ehmry/nix,rycee/nix,zimbatm/nix,shlevy/nix,zimbatm/nix,shlevy/nix,NixOS/nix,dezgeg/nix,NixOS/nix,dtzWill/nix,dezgeg/nix,vcunat/nix,layus/nix,shlevy/nix,peti/nix,rycee/nix,peti/nix,ehmry/nix,rycee/nix,dtzWill/nix,peti/nix,zimbatm/nix,dtzWill/nix,pikajude/nix,shlevy/nix,layus/nix