.circleci: remove directory

This removes building our doc at CircleCI. The doc is now build by the
new Murdock instance.

.circleci/config.yml

@@ -1,25 +0,0 @@
-# CircleCI configuration file used to build and get a preview of the generated
-# Doxygen documentation.
-# Note that the job on CircleCI takes around 30 minutes to complete because
-# copying +19k individual files to their cloud storage is very slow.
-
-version: 2
-
-jobs:
-  doc-build:
-    docker:
-      - image: riot/static-test-tools
-    steps:
-      - checkout
-      - run: make doc
-      - run:
-          command: |
-            cp -R doc/doxygen/html /doc
-      - store_artifacts:
-          path: /doc
-
-workflows:
-  version: 2
-  doc-build:
-    jobs:
-      - doc-build