Bug 15632: Koha::Patron::Messages - Remove AddMessage

 Bug 15632: Koha::Patron::Messages - Remove AddMessage

The AddMessage subroutine just
1. checked if all the parameters were passed
2. inserted the message in the DB
3. logged the action if needed

These 3 jobs are now done by the overloaded store method of
Koha::Patron::Message.

Test plan:
1/ Go on the "check out" page of a patron
2/ Add different new messages

Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Brendan A Gallagher <brendan@bywatersolutions.com>