Fix buildings losing their setLowLODElement link

[Flashmyname]

Summary

A building's setLowLODElement link was lost after setElementPosition, a dimension change or a building pool resize.

The cleanup #5440 added to Destroy also runs on those recreates, so that part is taken out and the null checks stay. Unlink still clears both sides on delete.

Motivation

Regression from #5440.

Test plan

With runcode:

crun a=createBuilding(1337,0,0,500) b=createBuilding(1337,0,0,510) setLowLODElement(a,b) setElementPosition(a,0,0,505) return getLowLODElement(a)==b, isElementLowLOD(b)
crun setLowLODElement(a,b) engineSetPoolCapacity("building",engineGetPoolCapacity("building")+5000) return getLowLODElement(a)==b, isElementLowLOD(b)
crun setLowLODElement(a,b) setElementDimension(localPlayer,1) setElementDimension(localPlayer,0) return getLowLODElement(a)==b, isElementLowLOD(b)

Master returns false, false on each, with the fix true, true.

Checklist

• Your code should follow the coding guidelines.
• Smaller pull requests are easier to review. If your pull request is beefy, your pull request should be reviewable commit-by-commit.