Consider removing citus.multi_shard_modify_mode

We should simply rely on `citus.max_adaptive_executor_executor` = `1` instead of `citus.multi_shard_modify_mode` = `sequential`.

This complicates the code.

In the first release, we can `WARN` the use that we're deprecating this setting and internally treat it as `citus.max_adaptive_executor_executor` = `1` , in the second release we can remove the setting.