If there's no value cell on the page, don't pass a previously unpacked
value cell to __rec_txn_read().
Simplify callers of __rec_txn_read() the WT_UPDATE list we're looking
at can be derived from the WT_INSERT/WT_ROW arguments.
We can't collapse removed key and value overflow items into a single
cell type: the code to find a row-store leaf page key's value has to
be able to distinguish between them because row-store leaf pages don't
store values that don't exist, so two adjacent keys (even removed
overflow keys), imply an empty value.