Well obviously the last two columns contain references to the two columns
that you delete. You need to either remove the references to the columns
that you want to delete, or, more simply, just hide them.

Ed
The last 2 columns are probably displaying #REF because they have formulas
in them that depend on values in the 2 columns that you are deleting.  You
need to either keep the first 2 columns (hide them if you want) or copy-paste
special-values the last 2 columns (right over the top of the existing
formulas if you want).
Will