Remove old walls from entities.yaml

[xsebek]

I want to replace the default walls with the new connecting wall from #1285 in entities.yaml.

This would make the new wall a more easily discoverable good default.

We can't quite get rid of all walls, as illustrated by the bucket brigade challenge map.

Originally posted by @byorgey in #2153 (comment)

In challenges that require custom walls the old walls would need to be included. This will make it explicit that the scenario needs custom wall rendering at the cost of being more verbose.