Consider removing `JapaneseExtended`

[robertbastian]

This calendar is slated for removal from CLDR: https://unicode-org.atlassian.net/browse/CLDR-11400 (CLDR will remove the eras, at which point the ICU4X calendars Japanese and JapaneseExtended will be identical). This type split is an ICU4X peculiarity, it's not supported by CLDR or locale extensions, and by extension the Temporal spec.

It probably makes sense to remove this sooner rather than later.

[nekevss]

Just to note from discussion on call.

This cannot currently be constructed from V8 or Boa, but it does have some overhead downstream due to match statements in downstream code.