Bug 36738: Don't copy date when repeating date fields

Bug 36738: Don't copy date when repeating date fields

Make the repeatable date patron attribute empty when repeating the
inputs to avoid date formatting problems (and keeping past date value
after repeating is not something we want)

Test plan:
1- create a patron attribute type with is a date and repeatable checked
(administration -> patron attribute types)
2- add a couple of patron attribute of type date, notice the repeated
inputs contain previous data
3- Apply the patch
4- redo step 2 and notice the repeated inputs are empty

Follow up for bug 32610 (https://bugs.koha-community.org/bugzilla3/show_bug.cgi?id=32610)

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>