Bug 26940: don't reset debarredcomment on patron update

Bug 26940: don't reset debarredcomment on patron update

Test plan:
1) Create a patron
2) In koha/members/moremember.pl add a manual restriction, with comment foobar
3) try to checkout, you have a message like
    Restricted: Patron's account is restricted with the explanation:
      foobar
4) Got to Edit patron, save
5) try to checkout, foobar is no more
    Restricted: Patron's account is restricted with the explanation:
6) Apply patch
7) Redo 1-4
8) try to checkout, foobar is there.

Signed-off-by: Andrew Fuerste-Henry <andrew@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Katariina Hanhisalo <katariina.hanhisalo@xamk.fi>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>