Bug 9788: (follow-up) for expirationdate in Letter.pm

Bug 9788: (follow-up) for expirationdate in Letter.pm

Pasting comment from the Bugzilla report:

Looking bit longer at this code, it is kind of strange to find it
there in the first place. Adding maxpickupdelay in Letters.pm should
not be there, but it is..

Also this date is not used normally in the default HOLD Available for
Pickup notice (that we are generating in this case). And if it would be
undef, the expiration date should imo be empty instead of today+0.
(before adding maxreservespickupdelay, you should test the allowexpire
pref first) So it is an (invisible) bug on its own.

Test plan:
See former patch. Kyle just discovered this bug, apparently by
deleting the maxpickupdelay pref..

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>
(cherry picked from commit a23d4181b1e756a61a036059b8b909d4f0d30537)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>