Check for CJK instead of CJKutf8.

Check for CJK instead of CJKutf8.

The specific test was introduced in ef6be5f4 because
CJKutf8 was relatively new (cf. lyx.org/trac/ticket/5386).

10 years on, CJKutf8 is an established part of the CJK bundle
and we can skip the special test for CJKutf8 to make the logic
considerabely simpler to read, maintain and debug.