Use shim for legacy client read_modify_write_row

The legacy client implements read_modify_write_row in a custom [AppendRow class](https://github.com/googleapis/python-bigtable/blob/3648d7000df4a108524e29a9c55639dc92c1c2cc/google/cloud/bigtable/row.py#L777). Conditions are built up, and then the request is executed as part of the [commit()](https://github.com/googleapis/python-bigtable/blob/3648d7000df4a108524e29a9c55639dc92c1c2cc/google/cloud/bigtable/row.py#L897) call

The new data client tried to avoid the complexity of these custom Row types, and its API closely resembles the gapic call. We can likely simple swap out the legacy client's [gapic call](https://github.com/googleapis/python-bigtable/blob/3648d7000df4a108524e29a9c55639dc92c1c2cc/google/cloud/bigtable/row.py#L938) for the [veneer one](https://github.com/googleapis/python-bigtable/blob/3648d7000df4a108524e29a9c55639dc92c1c2cc/google/cloud/bigtable/data/_async/client.py#L1541), and remove the [redundant validation](https://github.com/googleapis/python-bigtable/blob/3648d7000df4a108524e29a9c55639dc92c1c2cc/google/cloud/bigtable/row.py#L928)