Fix: revert broken nix standardization on lts/ghc-9.8.x

Reverts commit that broke flake.nix by adding import-tree outputs line without removing the old outputs body. This caused a Nix syntax error that has blocked CI for 11 days since 2026-03-04.

The nix directory standardization should be re-attempted properly via the existing MRs once the configuration files are adapted to the new module system.