Revert "Merge pull request #15384 from Shados/fix-preshell-terminfo"

This reverts commit 4e9833d9, reversing
changes made to 6194e9d8.

Setting TERMINFO prevents ncurses from reading TERMINFO_DIRS.  See
https://github.com/NixOS/nixpkgs/pull/15384#issuecomment-221205596