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/tests/unit/test_holidays.py

@@ -8,7 +8,6 @@ import pytest
 from app.utils.holidays import (
     UncachedYearError,
     cached_year_bounds,
-    get_holiday_name,
     get_red_days,
     get_swedish_holidays,
     is_klamdag,
@@ -114,33 +113,6 @@ class TestIsSwedishHoliday:
         assert not is_swedish_holiday(datetime.date(2026, 6, 1))
 
 
-class TestGetHolidayName:
-    """Test get_holiday_name function."""
-
-    def test_christmas_name(self) -> None:
-        """Get name of Christmas."""
-        name = get_holiday_name(datetime.date(2026, 12, 25))
-        assert name is not None
-        assert "Jul" in name
-
-    def test_non_holiday_returns_none(self) -> None:
-        """Non-holiday date returns None."""
-        name = get_holiday_name(datetime.date(2026, 6, 8))
-        assert name is None
-
-    def test_new_year_name(self) -> None:
-        """Get name of New Year's Day."""
-        name = get_holiday_name(datetime.date(2026, 1, 1))
-        assert name is not None
-        assert "Nyår" in name
-
-    def test_midsummer_name(self) -> None:
-        """Get name of Midsummer."""
-        name = get_holiday_name(datetime.date(2026, 6, 20))
-        assert name is not None
-        assert "Midsommar" in name
-
-
 # ---------------------------------------------------------------------------
 # De facto days (julafton, midsommarafton, nyårsafton) — cached-year source
 # ---------------------------------------------------------------------------
@@ -154,22 +126,13 @@ class TestDeFactoDaysAreNonWorking:
     def test_julafton_is_non_working(self) -> None:
         assert is_non_working_day(datetime.date(2027, 12, 24))
         assert is_swedish_holiday(datetime.date(2027, 12, 24))
-        name = get_holiday_name(datetime.date(2027, 12, 24))
-        assert name is not None
-        assert "Julafton" in name
 
     def test_nyarsafton_is_non_working(self) -> None:
         assert is_non_working_day(datetime.date(2027, 12, 31))
-        name = get_holiday_name(datetime.date(2027, 12, 31))
-        assert name is not None
-        assert "Nyårsafton" in name
 
     def test_midsommarafton_is_non_working(self) -> None:
         # Midsommardagen 2027 is Sat 2027-06-26 -> midsommarafton is Fri 6/25.
         assert is_non_working_day(datetime.date(2027, 6, 25))
-        name = get_holiday_name(datetime.date(2027, 6, 25))
-        assert name is not None
-        assert "Midsommarafton" in name
 
     def test_ordinary_weekday_is_still_working(self) -> None:
         assert not is_non_working_day(datetime.date(2027, 3, 2))  # a plain Tuesday