Hadrian: Drop remaining symlink traversal code from build scripts

This partly resolves #16325 (closed) (https://ghc.haskell.org/trac/ghc/ticket/16325).

As previously discussed in https://github.com/snowleopard/hadrian/issues/667, we do not need the symlink traversal code in build scripts. However, it appears we forgot to delete this code from our Stack-based build scripts, which led to placing all build artefacts in an unexpected location when using Hadrian in combination with symlink trees. This commit fixes this.