Delete miraged construction sites

[elexis]

It is possible to delete all buildings (given enough of capture points), unless the building hasn't been finished yet and is outside the players vision range (fog of war).

This is in particular annoying if one placed a civic center far away and wants to relocate it.

(Before #3107 the construction site heppened to be visible in the fog of war and was deletable until it was explored and inside the FoW again).