utf8.h: Simplify expressions

utf8.h: Simplify expressions

These expressions, while valid, are overly complicated in order to make
it easy to separate out problematic code points in the future, such as
surrogates.  But we made a decision in 5.12 to not go in that direction,
but to accept such problematic code points in general.  I  haven't
heard any cause to regret that decision; if we ever want to go back, the
blame log will easily allow us to.