pipeline: drop redundant shortcut

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.

osbuild/pipeline.py

@@ -291,11 +291,6 @@ class Pipeline:
         if not self.assembler:
             return results
 
-        # if the output is already in the store, short-circuit
-        if object_store.contains(self.output_id):
-            results["output_id"] = self.output_id
-            return results
-
         output = object_store.new()
 
         with build_tree.read() as build_dir, \