Remove CircleCI (#793)

Sytest has been switched to Buildkite. As such the `.circleci` folder has been removed.

.circleci/config.yml

@@ -1,32 +0,0 @@
-version: 2
-jobs:
-  dendrite:
-    docker:
-      - image: matrixdotorg/sytest-dendrite
-    working_directory: /src
-    steps:
-      - checkout
-
-      # Set up dendrite
-      - run: 
-          name: Build Dendrite
-          command: ./build.sh
-      - run: 
-          name: Copy dummy keys to root
-          command: |
-            mv .circleci/matrix_key.pem .
-            mv .circleci/server.key .
-      - run:
-          name: Run sytest with whitelisted tests
-          command: /dendrite_sytest.sh
-
-      - store_artifacts:
-          path: /logs
-          destination: logs
-      - store_test_results:
-          path: /logs
-workflows:
-  version: 2
-  build:
-    jobs:
-      - dendrite

.circleci/matrix_key.pem

@@ -1,5 +0,0 @@
------BEGIN MATRIX PRIVATE KEY-----
-Key-ID: ed25519:zXtB
-
-jDyHsx0EXbAfvM32yBEKQfIy1FHrmwtB1uMAbm5INBg=
------END MATRIX PRIVATE KEY-----

.circleci/server.key

@@ -1,52 +0,0 @@
------BEGIN PRIVATE KEY-----
-MIIJQwIBADANBgkqhkiG9w0BAQEFAASCCS0wggkpAgEAAoICAQCanRCqP11MLIQh
-nC26+A1oyBsFfH7auZ3pqE/WFDrCCIoc7ek7cF3fZU7q8OYI+Q9L5V8fobuLb6FB
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-+yaBExKfroZjsksctNAcAbgpuvhLLG8=
------END PRIVATE KEY-----

CONTRIBUTING.md

@@ -25,7 +25,9 @@ run](scripts/build-test-lint.sh).
 When a Pull Request is submitted, continuous integration jobs are run
 automatically to ensure the code builds and is relatively well-written. Checks
 are run on [Buildkite](https://buildkite.com/matrix-dot-org/dendrite/) and
-[CircleCI](https://circleci.com/gh/matrix-org/dendrite/).
+[CircleCI](https://circleci.com/gh/matrix-org/dendrite/). The Buildkite
+pipeline can be found in Matrix.org's [pipelines
+repository](https://github.com/matrix-org/pipelines).
 
 If a job fails, click the "details" button and you should be taken to the job's
 logs.