Remove unnecessary dir, work dir and step call in proto

This MR removes a few unnecessary proto fields.

The dir field shouldn't be part of the proto because it's the location of the downloaded step in the cache. That's a runtime detail that doesn't belong in the result.

The stepCall structure is an internal implementation of runner execution. It should be a Go struct instead. Nothing else references it in step.proto.

And workDir isn't used and I don't think it's necessary.