All callsites of `Pipeline.assemble()` already check early whether the
output-object exists in the store and then return it. Checking again in
`assemble()` will never catch anything (unless another stage would
happen to produce the same ID as the assembler as a side-effect).
It does seem useful to keep the shortcuts in `assemble()`, so other
callers would get the shortcut as well. However, this does not really
work well right now, since you want to skip the stage-compilation as
well, and `assemble()` is really just the last step of the job. Hence,
it really is the job of the pipeline-executor to check early.
With that in mind, lets drop this fast-path which has no effect in the
current setup.
18b16acd3f