Don't put inherited type-must-support extension in differential

While it isn't necessary _wrong_ to repeat an inherited type-must-support extension in a type, it is currently causing issues with the IG Publisher's snapshot generator.

See: [#IG creation > type-must-support is getting repeated @ 💬](https://chat.fhir.org/#narrow/channel/179252-IG-creation/topic/type-must-support.20is.20getting.20repeated/near/528770795)