There are no new releases anymore and the current version isn't
compatible with ruby 3.x.

As this feature wasn't really used a lot (Icelandic didn't even setup
inflections properly), it's probably not worth fighting for it, so lets
just drop it.

Related to diaspora#8369

`deep_merge!` modifies the hash of the default translation, and it looks
like `I18n.t` always returns the same instance, so after that, the
default stays translated. So lets duplicate the hash first, before
modifying it, this also helps because we also add more keys below, which
probably also shouldn't be added to the original.