ICU-22991 Remove unused private fIsSet

[FrankYFTang]

Remove unused private fIsSet from Calendar to save 24 bytes per Calendar object.

Java removed the isSet field at least 24 years ago.

Checklist

• Required: Issue filed: ICU-22991
• Required: The PR title must be prefixed with a JIRA Issue number. Example: "ICU-1234 Fix xyz"
• Required: The PR description must include the link to the Jira Issue, for example by completing the URL in the first checklist item
• Required: Each commit message must be prefixed with a JIRA Issue number. Example: "ICU-1234 Fix xyz"
• 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]

    fIsSet[field] = true; // Remove later

“later” is now!