Box: Fix tile with simultaneous floor and wall turfs in brig.

[warriorstar-orion]

What Does This PR Do

Removes a floor turf from a tile which is only meant to be a wall.

Why It's Good For The Game

One turf per tile is the rule.

Images of changes

Testing

Ran unit tests.

Changelog

:cl: fix: Fixed incorrect floor turf assignment in brig. /:cl: