ICU-22374 guard gCollationBinKey behind if !UCONFIG_NO_COLLATION

[srl295]

• no other callers when this is set

Checklist

• [x] Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-22374
• [x] Required: The PR title must be prefixed with a JIRA Issue number.
• [x] Required: The PR description must include the link to the Jira Issue, for example by completing the URL in the first checklist item
• [x] Required: Each commit message must be prefixed with a JIRA Issue number.
• [ ] Issue accepted (done by Technical Committee after discussion)

[srl295]

@richgillam OK to merge this and get it off the books?

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