Free Space by Unload Columns or Alter Table for hidden concat_attribute columns

I am trying to remove the concat attribute columns created by the join.  Tried these but didn't work

1) I can't seem to execute a unload syntax for a specific column, I can for a table but I can't do it for ANY columns let alone the hidden columns / concat_attribute.   UNLOAD <table_name> (<column_name>, ...)

2) ALTER TABLE <table_name>  <drop_column_clause> ::= DROP ( <column_name>[{, <column_name>} ...] ) I am typing in ALTER TABLE T_EMP DROP ($ENAME$DEPTID) this is not working on the hidden columns / concat_attribute but it is working on other columns.

Please see the screen shots of the SAP documention and the tests.