ICU-22160 clean up Calendar code

[FrankYFTang]

Remove unnecessary BuddhistCalendar::handleComputeMonthStart Remove unnecessary include

Checklist

• [X] Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-22160
• [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)
• [ ] Tests included, if applicable
• [ ] API docs and/or User Guide docs changed or added, if applicable

for ICU 73, not 72

[FrankYFTang]

no urgent need for 72. 73 should be fine

[pedberg-icu]

no urgent need for 72. 73 should be fine

Yes, we are after ICU 72 code freeze. This cannot be merged to main until after the maint/maint-72 branch is created.

[markusicu]

@FrankYFTang looks ready to merge for ICU 73.