Mining zone created even when building ghost can't be placed

A mining zone is created, and cannot be removed, if you try to place a mining tower in an invalid location.

1. Try to place mining tower over a tree or another object
2. Hover to place again, in a valid position nearby
3. Note that there is already a mining rectangle

A mining zone shouldn't be created if the tower can't be placed and if such a mining zone exists where it belongs to no tower it should be removed.

It does appear that the improper mining zone is cleaned up on game save loading