ICU-22421 Remove two unused internal methods in gregorian cal

[FrankYFTang]

Remove pinDayOfMonth() and yearLength(int32_t year) from GregorianCalendar.

These two methods are

1. Unused by any code inside ICU, not in source/{common,i18n,test}.
2. Marked as @internal in the header.
3. Wrap inside #ifndef U_HIDE_INTERNAL_API block in the header
4. In "protected:" section.
5. No ICU4J counterpart.

The yearLength(int32_t year) dup the functionality as handleGetYearLength of the same class and that one is the correct one to be keep and used..
There is another yearLength() w/o the year as parameter should NOT be removed and still needed internally.

Checklist

• Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-22421
• 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

[mihnita]

Thanks
M.