DOC: Remove old note about default unit
`to_datetime` docstring indicates that the default unit is 'ms', but it is 'ns'. This PR simply removes that comment so that there is no need to keep it in sync with the implementation.

- [ ] ~closes #xxxx (Replace xxxx with the Github issue number)~ NA
- [ ] ~[Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature~ NA
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [ ] ~Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.~ NA

thanks @gsganden 