ICU-22900 Remove deprecated SimpleNumber truncateStart

[sffc]

Checklist

• Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-22900
• Required: The PR title must be prefixed with a JIRA Issue number.
• Required: The PR description must include the link to the Jira Issue, for example by completing the URL in the first checklist item
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• Issue accepted (done by Technical Committee after discussion)
• Tests included, if applicable
• API docs and/or User Guide docs changed or added, if applicable

[markusicu]

This should ship in ICU 77

I don't see why this shouldn't go into 76. We are not removing anything that has ever been stable. The two functions that are to be removed went from draft ICU 73 to deprecated ICU 75, so anyone limiting themselves to stable API has never used them.

[richgillam]

I think Markus is right that we shouldn't jump the gun on promoting anything or do it in a special PR. I can take care of that the way I always do.

I'm agnostic on whether we should delete these APIs now or in 77, but I'm leaning toward Markus's position. The longer they're in there, the longer we have for people to start using them, making it worse when we get rid of them. And they're already marked as deprecated. I'm not sure I see the value in keeping them around.

[jira-pull-request-webhook]

Hooray! The files in the branch are the same across the force-push. 😃

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[markusicu]

Thanks -- please merge this today.