I don't believe it's possible for a key/value pair to be deleted without
instantiating the key, which makes this call to __wt_row_leaf_key_work
unnecessary; assert the fact.
Turns out truncation does delete without instantiating keys -- put
the code back and update the comment for clarity.
Merge pull request #799 from wiredtiger/overflow-instantiate

Remove call to instantiate a deleted overflow key during reconciliation.