python: remove post-processor staging

We shouldn't have to generate to an owl-bot-staging directory at all. Once we've removed the use of synthtool, we can generate directly into the right place. We then shouldn't have anything to clean up afterwards (and ideally we don't even need to figure out the repo root.)

Start by locating the Python generation path and tracing its uses of synthtool, owl-bot-staging, cleanup, and repository-root detection. Done means generation writes directly to the intended location without staging or post-generation cleanup; the issue does not name specific files or tests to run.

Written by the indexing model from the issue text.