Normally, on a table with a clustered index...
ALTER INDEX ALL ON mytable
REBUILD
reclaims space when a variable or fixed column is dropped.
Since you have no index, you could add and drop a clustered index to have the same effect.
CREATE CLUSTERED INDEX IX_MyIndex ON mytable (someColumn) ON [PRIMARY]
...
DROP INDEX IX_MyIndex ON mytable
Hmmm. My bad, I missed the second page of posts before I replied. Out of date, I suppose.