box: remove `cord_slab_cache` symbol from public API export

`cord_slab_cache` was not designed to offer a backward compatible API, and
leaving it exposed inevitably leads to errors like those highlighted in
#7124 and tarantool/memcached#96: hence, remove it from the public API
export.

Closes #7124

@TarantoolBot document
Title: `cord_slab_cache` was removed from public API export

The `cord_slab_cache` needs to be removed from the C API reference of the
fiber module.