ci(scripts): Remove buildkite scripts

Also removed a stack script that references a buildkite script. This will not
be be a problem because we no longer use stack. Note that there are a few more
scripts that reference to stack, which we can clean up later.

Author: sgillespie

ci/check-stack2nix.sh

@@ -2,15 +2,11 @@
 
 The `scripts` directory consists of the following directories:
 - [benchmarking](#benchmarking)
-- [buildkite](#buildkite)
 - [lite](#lite)
 - [shelley-from-scratch](#shelley-from-scratch)
 - [byron-to-alonzo](#byron-to-alonzo)
 - [plutus](#plutus)
 
-#### buildkite
-Contains scripts relevant to IOHK's CI.
-
 #### lite
 Contains scripts that can start various clusters and intended to be as simple as possible. Note that using the shelley only era testnet clusters breaks compatibility with some cli commands.