ICU-22991 Simplified Grego code

[FrankYFTang]

Use timeToFields instead of dayToFields if possible.

Checklist

• Required: Issue filed: ICU-22991
• Required: The PR title must be prefixed with a JIRA Issue number. Example: "ICU-1234 Fix xyz"
• 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

[jira-pull-request-webhook]

Notice: the branch changed across the force-push!

• icu4c/source/i18n/chnsecal.cpp is now changed in the branch
• icu4c/source/i18n/gregoimp.cpp is different
• icu4c/source/i18n/gregoimp.h is now changed in the branch
• icu4c/source/i18n/olsontz.cpp is now changed in the branch

View Diff Across Force-Push

~ Your Friendly Jira-GitHub PR Checker Bot

[jira-pull-request-webhook]

Notice: the branch changed across the force-push!

• icu4c/source/i18n/gregoimp.cpp is different

View Diff Across Force-Push

~ Your Friendly Jira-GitHub PR Checker Bot