chore: Refactor train-test split in plot_cqr_tutorial.py

[LacombeLouis]

Description

Fix issue on #477. Uses the same data and split for MapieRegressor and MapieQuantileRegressor.

Please remove options that are irrelevant.

• This change requires a documentation update

Checklist

• [ ] I have read the contributing guidelines
• [ ] I have updated the HISTORY.rst and AUTHORS.rst files
• [x] Linting passes successfully : make lint
• [x] Typing passes successfully : make type-check
• [x] Unit tests pass successfully : make tests
• [x] Coverage is 100% : make coverage
• [x] Documentation builds successfully : make doc