Remove call to instantiate a deleted overflow key during reconciliation.

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.

@michaelcahill: this call went in as part of #310, and there's wording there https://github.com/wiredtiger/wiredtiger/issues/310#issuecomment-9075481:
> Truncate is key to this failure: it's the only way, in the current engine, to modify a key/value pair without caching the key's value in memory.

I reviewed truncate, and I don't believe truncate can ever delete a row without instantiating its key.   If you agree (and can't think of any other path where we can remove a row without instantiating the key), I think we should make this change.