rfctr(msg): remove temporary new_msg.py (#3157)
**Summary**
Remove temporary `new_msg.py` module.
**Additional Context**
The rewrite of `partition_msg()` was placed in a separate file
`new_msg.py` to avoid a messy diff for code-review. This PR makes that
`new_msg.py` the new `msg.py`.
No code changes were made in the process.
