Remove obsoleted str_new_empty

since 58325daae3.

../string.c:1339:1: warning: ‘str_new_empty’ defined but not used [-Wunused-function]
 1339 | str_new_empty(VALUE str)
      | ^~~~~~~~~~~~~

string.c

@@ -1335,14 +1335,6 @@ rb_str_new_with_class(VALUE obj, const char *ptr, long len)
     return str_new0(rb_obj_class(obj), ptr, len, TERM_LEN(obj));
 }
 
-static VALUE
-str_new_empty(VALUE str)
-{
-    VALUE v = rb_str_new_with_class(str, 0, 0);
-    rb_enc_copy(v, str);
-    return v;
-}
-
 static VALUE
 str_new_empty_String(VALUE str)
 {