Remove non existent luautf8 charCodeAt extern (#11097)

std/lua/lib/luautf8/Utf8.hx

@@ -32,11 +32,6 @@ extern class Utf8 {
 	**/
 	static function sub(str:String, start:Int, ?end:Int):StringSub;
 
-	/**
-		Returns the character code at position `index` of `str`.
-	**/
-	static function charCodeAt(str:String, index:Int):Int;
-
 	/**
 		Looks for the first match of pattern in the string `str`.
 		If it finds a match, then `find` returns the indices of `str` where this