refactor: remove ouput_exprs from LogicalKafkaScan

[chenzl25]

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What's changed and what's your intention?

• Resolve Refactor: remove output_exprs from LogicalKafkaScan #16374

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[xxchan]

LGTM

[xxchan]

failed to run `e2e_test/source/basic/kafka.slt`
Caused by:
    statement failed: db error: ERROR: Panicked when handling the request: a stream key is expected but not exist, plan:
    LogicalSource { source: s20, columns: [id, first_name, last_name, email] }
    This is a bug. We would appreciate a bug report at:
      https://github.com/risingwavelabs/risingwave/issues/new?labels=type%2Fbug&template=bug_report.yml
    [SQL] create table s20 (
      id integer primary key,
      first_name varchar,
      last_name varchar,
      email varchar,
      gen_id integer as id+1
    ) with (
      connector = 'kafka',
      topic = 'debezium_log',
      properties.bootstrap.server = 'message_queue:29092'
    ) FORMAT DEBEZIUM ENCODE JSON
    at e2e_test/source/basic/kafka.slt:305

[chenzl25]

LGTM

I need to remove output_row_id_index from the source as well.

[chenzl25]

cc @xxchan I underestimated the refactoring of source 🥵 , because it is deeply coupled with the generated columns and watermark filter. Let me change this PR just for LogicalKafkaScan