ẞ is both required and non-required

[thlinard]

According to https://github.com/googlefonts/lang/blob/main/Lib/gflanguages/data/languages/de_Latn.textproto, ẞ is both in base and in not_required. It should only be in one category, and according to #152, in the base category.

ẞ should therefore be removed from "non_required".