String#index should no longer take integer argument

src/string.c

@@ -1389,17 +1389,6 @@ mrb_str_index_m(mrb_state *mrb, mrb_value str)
   }
 
   switch (mrb_type(sub)) {
-    case MRB_TT_FIXNUM: {
-      mrb_int c = mrb_fixnum(sub);
-      mrb_int len = RSTRING_LEN(str);
-      unsigned char *p = (unsigned char*)RSTRING_PTR(str);
-
-      for (;pos<len;pos++) {
-        if (p[pos] == c) return mrb_fixnum_value(pos);
-      }
-      return mrb_nil_value();
-    }
-
     default: {
       mrb_value tmp;