Remove unnecessary roofs from steel mill for shadowcasting performance

[SurFlurer]

Summary

none

Purpose of change

Address #72508

There are many unnecessary roofs in the steel mill. Roofs mark map squares as indoors, thus weather sight penalty isn't applied on those squares' transparency values. But in this case, the map structure creates too much fluctuations in transparency, thus shadowcasting starts struggling.

Describe the solution

Remove most roofs that are supposed to be outdoors.

Describe alternatives you've considered

1. Improve shadowcasting performance: I tried and came to the conclusion that I'm not capable enough to do that.
2. Improve the algorithm of identifying whether a map square should be subject to sight penalty: this really should be done ( in hope of fixing 3D shadowcasting bug for weather with sight_penalty != 1.0 #65205 ), but it takes time more than this simple solution.

Testing

Newly generated steel mills no longer suffer from degraded shadowcasting performance. Note that already explored steel mills aren't salvageable.

For comparison, below are pictures of the second floor of the steel mill, before and after the change.

before:

After:

Additional context