Update structures.yaml

Touched up descriptions with help from Phrohdoh and Maunz.
Removed bib from Construction Yard and Helipad.

Construction Yard bib was exploitable, when you surround your CY with walls you can still para-drop or chronoshift units onto the bib inside of the walls.

Helipad doesn't need a bib and it should have a smaller footprint then an airfield.

Most other buildings don't need the bib either as its a carry-over from the C&C95 days when buildings had no space in between them(to prevent boxing yourself in) and it looks terrable IMO, but I digress.