grim/convey

Remove the convey/extend task as it's normal usage is already covered by plan/stage environment variables. It's corner case, expanding variables, is rather exotic and not something that comes up often.
# This test just ensure that we can set the workdir to ${CONVEY_WORKSPACE}
# successfully
tasks:
basic:
type: docker/run
image: alpine:edge
workdir: ${CONVEY_WORKSPACE}
script:
- pwd
- ls
plans:
default:
stages:
- tasks: basic