Bug 19463: (follow-up) Cosmetic changes

Bug 19463: (follow-up) Cosmetic changes

String::Random version 0.26 (on Jessie) does not yet support the rand_gen
parameter (0.27 does, newest is 0.29 on CPAN now).
So alt_rand is only used in determining the size on Jessie.
That might be enough though.
Adding a documention line in this regard.

Removing the obsolete max parameter.

Note: I timed alt_rand for the creation of a new Bytes::Random::Secure
object each time. But each call is about 0.1 milliseconds. So that
should be fine.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>