ci: remove Drone CI

.drone.yml

@@ -1,31 +0,0 @@
----
-kind: pipeline
-type: exec
-name: abacus checks
-
-steps:
-- name: flake check
-  commands:
-  - nix flake check
-
-- name: package check
-  commands:
-  - nix build
-
-- name: notifiy
-  commands:
-  - nix run github:ambroisie/matrix-notifier
-  environment:
-    ADDRESS:
-      from_secret: matrix_homeserver
-    ROOM:
-      from_secret: matrix_roomid
-    USER:
-      from_secret: matrix_username
-    PASS:
-      from_secret: matrix_password
-  when:
-    status:
-      - failure
-      - success
-...