Serials: Don't force a Receive Unit Template in Alt Serials Control

Serials: Don't force a Receive Unit Template in Alt Serials Control

Mary Llewellyn reported that she used to be able to use serials and
perform copy-less receiving without any value in this field.

The requirement that a value be set in the Receive Unit Template field
of serial distributions was added (by me) in 2.1 to the alternate serials
control interface, but is not enforced at lower levels.

I now believe that requiring a value here must have been a ham-handed
way to avoid a difficult-to-troubleshoot issue in copy-FUL receiving,
but copy-LESS serials receiving workflows indeed don't need this, so
this commit removes the requirement.

Signed-off-by: Lebbeous Fogle-Weekley <lebbeous@esilibrary.com>
Signed-off-by: Ben Shum <bshum@biblio.org>