ssh-tpm-agent: drop override once upstream cuts a release including PR #114

[Mic92]

nixosModules/ssh-tpm-agent.nix currently pins Foxboron/ssh-tpm-agent@a3cde87 to get ssh-tpm-add -c and the SSH_ASKPASS_PROMPT fix from Foxboron/ssh-tpm-agent#114.

Once a tagged release containing that commit lands in nixpkgs, drop the overrideAttrs and go back to plain pkgs.ssh-tpm-agent.