Bug 20444: Use Koha::Patron::Attribute::Types object for getting patron attributes...

Bug 20444: Use Koha::Patron::Attribute::Types object for getting patron attributes in letter.pl

Test plan:
0) Apply the patch
1) Do not have any patron attribute types defined
2) Go to Tools -> Notices and slips
3) Edit any slip/letter which uses table borrower
4) Confirm the editing works, and the slip/letter itself works
5) Add some patron attributes types and define them a value for any
patron
6) Go to Tools -> Notices and slips and edit any slip/letter which uses
table borrower
7) Confirm, the attributes are listed in available fields
8) Confirm the slip/letter is working when you add a attribute to it

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