Remove visualization_old (mesa-viz-tornado) (projectmesa#2133)

Removing the old, legacy visualisation. It will still be available in 2.3.x.

Checkout the Visualization Tutorial (https://mesa.readthedocs.io/en/latest/tutorials/visualization_tutorial.html) to migrate to the new visualisation.

Author: rht

docs/index.md

@@ -83,7 +83,6 @@ Best Practices <best-practices>
 How-to Guide <howto>
 API Documentation <apis/api_main>
 Mesa Packages <packages>
-tutorials/adv_tutorial_legacy.ipynb
 ```
 
 # Indices and tables

docs/mesa.md

@@ -1,11 +1,5 @@
 # mesa package
 
-## Subpackages
-
-```{toctree}
-mesa.visualization_old
-```
-
 ## Submodules
 
 ## mesa.agent module