Minor: adjust visibility of LogicalPlan:: apply_subqueries and LogicalPlan::map_subueries

[alamb]

Which issue does this PR close?

Part of #9994

Rationale for this change

These methods were introduced in #9913 by @peter-toth but I think they were part of a previous implementation and unused.

What changes are included in this PR?

1. Mark apply_subqueries and map_subueries pub
2. Remove the (unused) transform_* functions

Are these changes tested?

CI

Are there any user-facing changes?

No (as the removed functions have not been released)

[peter-toth]

1. looks good to me, but regarding 2. I'm not sure we need to remove those APIs: #9913 (comment). The transform_..._with_subquery() versions are indeed not in use yet, but can be useful in the future.

[alamb]

1. looks good to me, but regarding 2. I'm not sure we need to remove those APIs: Consistent LogicalPlan subquery handling in TreeNode::apply and TreeNode::visit  #9913 (comment). The transform_..._with_subquery() versions are indeed not in use yet, but can be useful in the future.

Makes sense -- I will make a new PR that makes them pub / adds docs

[alamb]

#9998 for making them pub. I will document them as a follow on PR