[libc++] Remove NaCl support from __cxx03 headers (#219787)

Completes the NaCl cleanup started in #148983 by removing the remaining
references from the frozen `__cxx03/` headers.

Changes:

* Remove the `_LIBCPP_USING_NACL_RANDOM` comment block and
`#elif defined(__native_client__)` branch from
`include/__cxx03/__config`
* Remove `defined(__pnacl__)` from the `#if` condition in
  `include/__cxx03/limits`

The `_LIBCPP_USING_NACL_RANDOM` macro was defined but never consumed;
its
previous consumer in `src/random.cpp` was already removed in #148983.

Fixes #219588

GitOrigin-RevId: 989f9333f9ce9ee0285a5fb9c57fa5aecb75b4ed