DOC: Remove old warning from dsintro.rst
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master | flake8 --diff`
- [ ] whatsnew entry

The warning is about something that have been fixed for almost 3 years. Every time a new user excited about pandas start reading the docs, they have to waste brain-cycles ignoring that big red warning bubble.
