Fixes cas targeting incorrect signals

[vero5123]

About the pull request

Closes #6447, untested.

Explain why it's good for the game

bug bad

Changelog

🆑
fix: fixes cas being able to target shipside signals
/:cl:

[harryob]

actually, is there a particular reason this isn't in the valid_signal() check?

[vero5123]

actually, is there a particular reason this isn't in the valid_signal() check?

the valid_signal does a few checks like preventing the signal from being triggered from pockets. It's called multiple times during the process of executing the firemission, I just thought it wasn't really necessary to make sure it was ground z level for each check each time and we could just simply do the initial ground level check whenever when we try and get the target list for the dropship since all the subsequent checks come after that, maybe a few rare edge cases that wouldn't be covered but I can change it if necessary. Also, i'm probably going to refactor this altogether once I add the cas sim back so it will be changed regardless soon.