Remove unused replacement from Visibility.maintain

[goderbauer]

While reviewing #122585 I noticed that the replacement parameter on Visibility.maintain doesn't make any sense. It is always ignored because the constructor forces maintainState to true:

flutter/packages/flutter/lib/src/widgets/visibility.dart

Lines 139 to 140 in e0ea39f

	/// If this property is true, an [Offstage] widget is used to hide the child
	/// instead of replacing it with [replacement].

Therefore, this PR removed the replacement parameter from Visibility.maintain.

[flutter-dashboard]

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 (don't just cc him here, he won't see it! He's on Discord!).

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[tvolkert]

LGTM, but dartdocs for Visibility.maintain() need to be updated too.

[goderbauer]

Ah, right. Updated by removing that line altogether. Since we don't support non-nullsafe mode anymore, the note about child not being null is also redundant.

[Hixie]

test-exempt: removing code