python@2: delete (#49796)

EOL 1 January 2020.
We gave it 1 month more to live so that people had time to migrate.
All in all, developers had 11 years to do their migration.

You can use the `brew extract` command and maintain python@2 in your own
tap if necessary:
https://docs.brew.sh/How-to-Create-and-Maintain-a-Tap