chore: stale setup.py, Makefile and MANIFEST.in leftovers, and two reST READMEs outside docs/ #1773

Describe the bug
Stale build and packaging leftovers:

• setup.py:170: pcapkit_bdist_wheel.run refactors os.path.join(self.dist_dir, "pcapkit"), but dist_dir holds built wheels, not a package tree. It also runs after the wheel is written, so it can never change the wheel.
• setup.py:50 and :73: the f2format and poseur failures both log "assignment expression backport". Only walrus (:62) is that; f2format back-ports f-strings and poseur back-ports positional-only parameters.
• make profile runs $(MAKE) -C test profile and fails with test: No such file or directory, because test/Makefile was deleted in 78e28d811. MANIFEST.in:12 prune test names the same missing directory.
• examples/benchmark/README.rst and examples/legacy_smoke/README.rst are the only reST files outside docs/, against the Markdown-outside-docs ruling. They ship through MANIFEST.in's global-include *.rst.

Expected behavior
Remove or correct each of these, and keep the sdist contents as they are now.

Additional context
Found by the #1103 repo-slice worker, and each item was verified by a Sonnet review. The README rename needs the MANIFEST.in change in the same PR.