ICU-22893 Remove support for Unsupported, Private & Reserved constructs

[mihnita]

• Corresponds to Remove forward-compatibility promise and all reserved & private syntax message-format-wg#883
• Also removes some unused imports

Checklist

• Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-22893
• 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
• Required: Each commit message must be prefixed with a JIRA Issue number.
• Issue accepted (done by Technical Committee after discussion)
• Tests included, if applicable
• API docs and/or User Guide docs changed or added, if applicable

[echeran]

LGTM

[mihnita]

Thank you very much!