Retire regional overview

The [regional overview](https://github.com/quintel/scenario-tools/blob/master/scripts/regional_overview.py) functionality is outdated and should be retired. The corresponding [documentation](https://github.com/quintel/documentation/blob/master/docs/main/scenario-tools/regional-overview.md) should then also be removed.

@kndehaan or @kaskranenburgQ can either of you pick this up?