Attempt to delete more duplicate rows in receipts_linearized table. (#14915)

The previous assumption was that the stream_id column was unique
(for a room ID, receipt type, user ID tuple), but this turned out to be
incorrect.

Now find the max stream ID, then map this back to a database-specific
row identifier and delete other rows which match the (room ID, receipt type,
user ID) tuple, but *not* the row ID.