Remove mrb_str_sublen() as it is not used.

include/mruby/string.h

@@ -55,7 +55,6 @@ mrb_value mrb_str_buf_cat(mrb_state *mrb, mrb_value str, const char *ptr, int le
 
 char *mrb_string_value_cstr(mrb_state *mrb, mrb_value *ptr);
 char *mrb_string_value_ptr(mrb_state *mrb, mrb_value ptr);
-int mrb_str_sublen(mrb_state *mrb, mrb_value str, int pos);
 int mrb_str_offset(mrb_state *mrb, mrb_value str, int pos);
 mrb_value mrb_str_dup(mrb_state *mrb, mrb_value str); /* mrb_str_dup */
 mrb_value mrb_str_intern(mrb_state *mrb, mrb_value self);

src/string.c

@@ -1661,15 +1661,6 @@ mrb_str_rindex(mrb_state *mrb, mrb_value str, mrb_value sub, mrb_int pos)
   }
 }
 
-#ifdef INCLUDE_ENCODING
-/* byte offset to char offset */
-int
-mrb_str_sublen(mrb_state *mrb, mrb_value str, long pos)
-{
-  return pos;
-}
-#endif //INCLUDE_ENCODING
-
 /* 15.2.10.5.31 */
 /*
  *  call-seq: