fix(web): fix `KeyboardStub.validateForCustomKeyboard`
This change fixes a misleading error message in `validateForCustomKeyboard` - `KR` is not the region as the error implies - furthermore it doesn't make sense to check `KR` since it is the optional [keyboard registration function](https://help.keyman.com/developer/engine/web/18.0/reference/interface/registerKeyboard), so this change removes it.
Keyman makes it possible for you to type in any language
on Windows, macOS, Linux, iPhone, iPad, Android tablets and phones, and even
instantly in your web browser. Create keyboard layouts with Keyman Developer and
share them with the community in the keyboards
repository. The Keyman
Community have so far contributed
well over 1,000 keyboard layouts for over 2,500 languages!
License
Copyright (c) SIL Global.
Keyman is an open source project distributed under the MIT license.