The class sun.font.CharSequenceCodePointIterator is not used anywhere in the JDK.

Also it has (at least) one bug which would
cause an exception if a hi surrogate were the last char

    public int next() {
        if (index < text.length()) {
            char cp1 = text.charAt(index++);
            if (Character.isHighSurrogate(cp1) && index < text.length()) {
                char cp2 = text.charAt(index+1); /// <<< Should not be "+1"
                if (Character.isLowSurrogate(cp2)) {
                    ++index;
                    return Character.toCodePoint(cp1, cp2);
                }
            }
            return cp1;
        }
        return DONE;
    }

We could fix it, but it wouldn't help anyone and it would be hard to test.
It might be best to just delete the class.

Since we've not needed it in 2 decades I doubt we'll ever need it.