Drop obsolete .drone.yml

Drone CI may make sense in the future when I set up a private
instance that supports FUSE, but for now it provides no advantages
to Travis CI.

.drone.yml

@@ -1,6 +0,0 @@
-build:
-  image: golang
-  commands:
-    - go get
-    - go build
-    - go test ./cryptfs