Polyfill: Make gregory calendar return undefined week numbers

This is currently unspecified behaviour, and is to be specified later in
tc39/proposal-intl-era-monthcode#15

Closes: #3096

Author: ptomato

polyfill/lib/calendar.mjs

@@ -75,22 +75,20 @@ function arrayFromSet(src) {
 }
 
 function calendarDateWeekOfYear(id, isoDate) {
-  // Supports only Gregorian and ISO8601 calendar; can be updated to add support for other calendars.
+  // Supports only ISO8601 calendar; can be updated to add support for other calendars.
   // Returns undefined for calendars without a well-defined week calendar system.
   // eslint-disable-next-line max-len
   // Also see: https://github.com/unicode-org/icu/blob/ab72ab1d4a3c3f9beeb7d92b0c7817ca93dfdb04/icu4c/source/i18n/calendar.cpp#L1606
-  if (id !== 'gregory' && id !== 'iso8601') {
-    return { week: undefined, year: undefined };
-  }
+  if (id !== 'iso8601') return { week: undefined, year: undefined };
   const calendar = impl[id];
   let yow = isoDate.year;
   const { dayOfWeek, dayOfYear, daysInYear } = calendar.isoToDate(isoDate, {
     dayOfWeek: true,
     dayOfYear: true,
     daysInYear: true
   });
-  const fdow = id === 'iso8601' ? 1 : calendar.helper.getFirstDayOfWeek();
-  const mdow = id === 'iso8601' ? 4 : calendar.helper.getMinimalDaysInFirstWeek();
+  const fdow = 1;
+  const mdow = 4;
 
   // For both the input date and the first day of its calendar year, calculate the day of week
   // relative to first day of week in the relevant calendar (e.g., in iso8601, relative to Monday).
@@ -1687,12 +1685,6 @@ const helperGregory = ObjectAssign(
       if (era === 'b') era = 'gregory-inverse';
       if (era === 'a') era = 'gregory';
       return { era, eraYear };
-    },
-    getFirstDayOfWeek() {
-      return 1;
-    },
-    getMinimalDaysInFirstWeek() {
-      return 1;
     }
   }
 );