Remove duplicates header files in `src/pool.c`

These are included in `mruby.h`.

As a background, if `enable_cxx_abi` is specified, the macro that
defines the maximum value in `stdint.h` is undefined depending on the
environment.

- Confirmed with gcc on FreeBSD 12.0 and mingw32-gcc available on
  FreeBSD.
- `INTPTR_MAX`, `INT64_MAX` and `UINT64_MAX` are defined by `cstdint`
  added in C++11. But `toolchains :gcc` adds `-std=c++03`, so the macros
  are not defined.
- To have these defined in `C++03`, `__STDC_CONSTANT_MACROS` must be
  defined in advance. This is already done by `mruby.h`.

src/pool.c

@@ -4,8 +4,6 @@
 ** See Copyright Notice in mruby.h
 */
 
-#include <stddef.h>
-#include <stdint.h>
 #include <string.h>
 #include <mruby.h>