Remove stubs for Babel (not until August 2023)

[`Babel`](https://github.com/python-babel/babel) has just incorporated typeshed's stubs as inline annotations and added a `py.typed` file (see https://github.com/python-babel/babel/pull/934). These changes were released in [`v2.12.0`](https://github.com/python-babel/babel/releases/tag/v2.12.0) on Feb 28, 2023.