Remove ifndef FIPS_MODE from rand_unix.c

This will never be the case for 1.1.1 so removed.

Fixes: comment 1 of #9757

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/9762)