Remove arc4random() OpenBSD support

Remove arc4random() OpenBSD support

The arc4random() support for OpenBSD does not build. The intention was
to include any portability code so that the library could be easily
ported to such systems, but it makes little sense to build it on OpenBSD
where most of the functionality will be already present, or the software
would have been ported anyway.

[guillem@hadrons.org: Reword commit message to add rationale. ]

Closes: !15
Signed-off-by: Guillem Jover <guillem@hadrons.org>