ci: remove .buildkite

We do not use buildkite since 11/2021. Remove the folder with buildkite
specific files.

Signed-off-by: Anas Nashif <[email protected]>

Author: nashif

.buildkite/daily.yml

@@ -15,7 +15,6 @@
 
 /.github/				  @nashif
 /.github/workflows/                       @galak @nashif
-/.buildkite/				  @galak
 /MAINTAINERS.yml                          @MaureenHelm
 /arch/arc/                                @abrodkin @ruuddw @evgeniy-paltsev
 /arch/arm/                                @MaureenHelm @galak @ioannisg

.buildkite/hooks/post-command

@@ -1790,7 +1790,6 @@ CI:
         - galak
     files:
         - .github/
-        - .buildkite/
         - scripts/ci/
         - .checkpatch.conf
         - scripts/gitlint/