spectral: remove salishan

There's no Haskell code in `spectral/salishan`, so let's just remove it.

Signed-off-by: Michal Terepeta <michal.terepeta@gmail.com>

Test Plan: build & run

Reviewers: bgamari

Subscribers: snowleopard

Differential Revision: https://phabricator.haskell.org/D3321