Description

This follows 21cfed3 by removing the childToSlot pattern usages
where possible - namely, RenderObject subclasses. There are still
usages of the pattern in RenderObjectElement subclasses, where they
use the pattern to efficiently forget a child in forgetChild().

Related Issues

#63269

Tests

Test exemption: this just removes code. The existing tests passing validates the change.

Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I signed the [CLA].
• I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I updated/added relevant documentation (doc comments with ///).
• All existing and new tests are passing.
• The analyzer (flutter analyze --flutter-repo) does not report any problems on my PR.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Did any tests fail when you ran them? Please read [Handling breaking changes].

• No, no existing tests failed, so this is not a breaking change.
• Yes, this is a breaking change. If not, delete the remainder of this section.