Remove deprecated child parameter for NestedScrollView's overlap managing slivers

[Piinks]

Description

These were deprecated in #44283, while the new breaking change policy was going into place.
Checking that clients are migrated and that a migration guide will be available for when this is finally removed.

Checklist

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Breaking Change

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[fluttergithubbot]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption to this rule, contact Hixie on the #hackers channel in Chat.

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[goderbauer]

LGTM