remove unused buildkite pipeline (#3453)

the buildkite build is archived

.buildkite/pipeline.yaml

@@ -1,39 +0,0 @@
-steps:
-  - label: ":snake: Build swagger definitions for matrix.org"
-    command:
-      # Install the python dependencies necessary to build the spec
-      - python3 -m venv env && . env/bin/activate
-      - pip install -r scripts/requirements.txt
-      # Build the spec
-      - scripts/generate-matrix-org-assets
-    artifact_paths:
-      - assets.tar.gz
-    plugins:
-      - docker#v3.7.0:
-          image: python:3.9
-
-  - label: "rebuild matrix.org"
-    trigger: "matrix-dot-org"
-    async: true
-    branches: "main"
-
-  - label: ":books: Build the spec"
-    command:
-      # Install package dependencies
-      - apk add nodejs npm git hugo
-      # Install the node dependencies necessary to build the spec
-      - npm i
-      # Pull all git submodules, required for the hugo theme
-      - git submodule update --init --recursive
-      # Pull current proposal information
-      - npm run get-proposals
-      # Build the spec, will build to './spec'
-      # Set the baseURL as we're deploying to https://spec.matrix.org/unstable
-      - hugo --baseURL "/unstable" -d "spec"
-      # Compress the result and make it available as an artifact
-      - tar -czf spec.tar.gz spec
-    artifact_paths:
-      - spec.tar.gz
-    plugins:
-      - docker#v3.7.0:
-          image: alpine