[SPARK-51326][CONNECT] Remove LazyExpression proto message

[ueshin]

What changes were proposed in this pull request?

Removes LazyExpression proto message.

As any feature using this proto message is not released yet, it's safe to remove it from master and branch-4.0.

Why are the changes needed?

Discussed offline and found out that Spark Connect should work without it.

Does this PR introduce any user-facing change?

No.

How was this patch tested?

The existing tests should pass.

Was this patch authored or co-authored using generative AI tooling?

No.

[ueshin]

As "Protobuf breaking change detection and Python CodeGen check" failed with:

Error: Previously present message "LazyExpression" was deleted from file.
Error: Previously present field "22" with name "subquery_expression" on message "Expression" was deleted.
Error: Field "21" with name "subquery_expression" on message "Expression" changed option "json_name" from "lazyExpression" to "subqueryExpression".
Error: Field "21" with name "subquery_expression" on message "Expression" changed type from "spark.connect.LazyExpression" to "spark.connect.SubqueryExpression".
Error: Field "21" on message "Expression" changed name from "lazy_expression" to "subquery_expression".
Error: buf found 5 breaking changes.

#50094 should show it's not broken compared to 3.5.

• https://github.com/ueshin/apache-spark/actions/runs/13555973829/job/37890264016