Minor QOL things for xeno special structure construction

[SabreML]

About the pull request

1. Updates the 'help' message descriptions for xeno special structures, and refactors them a bit to use the pre-existing /datum/construction_template/var/description variable rather than a global proc.
2. Adds a 'remaining' counter to the special structure construction message to let players know how many can still be built.
3. Removes a redundant check in code/modules/mob/living/carbon/xenomorph/abilities/general_powers.dm.
4. Removes a couple of unused special structure defines. ("Unused" as in there was no structure datum tied to them, just a few references.)

Explain why it's good for the game

Just a little QOL thing, because I noticed that the limit and sometimes the function of some structures isn't very widely known.

Testing Photographs and Procedure

Screenshots & Videos

Before:

Help message:

Construction messages:

After:

Help message:

Construction messages:

Changelog

🆑
add: Updated the 'help' message for xeno special structure construction.
qol: Added a 'remaining' counter when constructing special structures to let players know how many can still be built.
/:cl:

[cm13-github]

This pull request has conflicts, please resolve those before we can evaluate the pull request.

[cm13-github]

Conflicts have been resolved. A maintainer will review the pull request shortly.