This PR removes the tests for the legacy checkbox syntax. A separate PR
will be used to remove the implementation for the legacy syntax.

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Co-authored-by: ionitron <hi@ionicframework.com>

This pull request includes the changes to remove the `legacy` property
for the checkbox as part of
#29043. That pull
request specifically focuses on updating tests to remove any legacy
checkbox usage. The internal ticket suggested separating these changes
into individual pull requests. Please refer to the mentioned pull
request for a detailed description of the combined changes from both
pull requests. This will be merged into that pull request upon approval.