Fix GhostScript detection (Trac #15856)

The option `confdir` (used in GhostScript test) was set correctly
via `--config` in `test.mk` and incorrectly via `config/ghc`.
AFAICT, some time ago this was working because the
incorrect assignment was done first, and later it broke.

Hardian doesn't pass `confdir`. I removed `confdir` and use
`config.top` to determine the directory of the
`good.ps` and `bad.ps` files. This is simpler.

I also removed some redundant assignments in `config/ghc`.

Test Plan: manually set config.have_profiling and make test

Reviewers: bgamari

Subscribers: rwbarton, carter

GHC Trac Issues: #15856

Differential Revision: https://phabricator.haskell.org/D5298