Remove the buildkite pipeline (#763)

New repo: https://github.com/matrix-org/pipelines/

.buildkite/pipeline.yaml

@@ -1,49 +0,0 @@
-steps:
-  - command:
-    # https://github.com/golangci/golangci-lint#memory-usage-of-golangci-lint
-    - "GOGC=20 ./scripts/find-lint.sh"
-    label: "\U0001F9F9 Lint / :go: 1.12"
-    agents:
-      # Use a larger instance as linting takes a looot of memory
-      queue: "medium"
-    plugins:
-      - docker#v3.0.1:
-          image: "golang:1.12"
-
-  - wait
-
-  - command:
-    - "go build ./cmd/..."
-    label: "\U0001F528 Build / :go: 1.11"
-    plugins:
-      - docker#v3.0.1:
-          image: "golang:1.11"
-    retry:
-      automatic:
-        - exit_status: 128
-          limit: 3
-
-  - command:
-    - "go build ./cmd/..."
-    label: "\U0001F528 Build / :go: 1.12"
-    plugins:
-      - docker#v3.0.1:
-          image: "golang:1.12"
-    retry:
-      automatic:
-        - exit_status: 128
-          limit: 3
-
-  - command:
-    - "go test ./..."
-    label: "\U0001F9EA Unit tests / :go: 1.11"
-    plugins:
-      - docker#v3.0.1:
-          image: "golang:1.11"
-
-  - command:
-    - "go test ./..."
-    label: "\U0001F9EA Unit tests / :go: 1.12"
-    plugins:
-      - docker#v3.0.1:
-          image: "golang:1.12"