Delete date helpers nothing calls

A hand-rolled month-arithmetic module survived the move to dateutil with no
caller left but its own tests, and two predicates alongside it were never
wired up at all. An error import was also being deferred inside a function
for a circular dependency that does not exist.

backend/app/utils/holidays.py

@@ -183,12 +183,6 @@ def is_swedish_holiday(date: datetime.date) -> bool:
     return date in holidays_for_year
 
 
-def get_holiday_name(date: datetime.date) -> str | None:
-    """Get the name of a Swedish public holiday, or None if not a holiday."""
-    holidays_for_year = get_swedish_holidays(date.year)
-    return holidays_for_year.get(date)
-
-
 def is_non_working_day(date: datetime.date) -> bool:
     """True if `date` is a weekend day, a public holiday, or a de facto day
     off (julafton, midsommarafton, nyårsafton, ...).