Deprecate unused `ButtonStyleButton.iconAlignment` property (#160023)

Fixes [Deprecate unused `ButtonStyleButton.iconAlignment`
property](https://github.com/flutter/flutter/issues/159782)

Refactor done in https://github.com/flutter/flutter/pull/158503 makes
`ButtonStyleButton.iconAlignment` redundant. In this PR I'm marking it
as deprecated without migration guide. However, it may possible to
remove it altogether.


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