Fix removing files on update

During updating, if the `#remove_<column>` was assigned, then
carrierwave_backgrounder would prevent deletion of that photo.

carrierwave_backgrounder doesn't have the functionality of deleting
files in the background (nor should it, this is usually a very cheap operation).
But it prevents the removal of the file, as if it's going to do it
in the background. We add here the additional conditional: "if the
file is scheduled for removal, then don't try any background business".