Remove Pluribus deployment

[assistant-ngalaiko]

Summary

• Remove Pluribus services, executor user/group, ingress routes, and backup registration.
• Remove deployment configuration, flake input and its unused dependency, offline check, and email documentation.
• Leave runtime data untouched; removal takes effect after merge and redeploy.

Validation

• nix fmt (no formatting changes)
• nix eval --raw .#packages.x86_64-linux.exedev.drvPath
• nix build .#checks.x86_64-linux.formatting --no-link
• git diff --check
• No remaining Pluribus references in the repository.