Polyfill: Don't print era in PlainMonthDay formatting

See the change in #3175
for details. `era` should not be added to the default options for
PlainMonthDay, because [[era]] isn't copied in step 12.b of
GetDateTimeFormat when defaults is MONTH-DAY.

The spec is correct here, this brings the reference code into alignment
with it.

Author: ptomato

polyfill/lib/intl.mjs

@@ -445,6 +445,7 @@ function monthDayAmend(originalOptions) {
     full: { month: 'long', day: 'numeric' }
   };
   const options = amend(originalOptions, {
+    era: false,
     year: false,
     hour: false,
     minute: false,