Bug 20603: Remove ModNote, getcredits and getrefunds from C4::Accounts

Bug 20603: Remove ModNote, getcredits and getrefunds from C4::Accounts

Test plan:
0) Do not apply the patch
1) Run these 3 commands
    git grep ModNote
    git grep getcredits
    git grep getrefunds
all should return occurances only in C4/Accounts.pm - the sub definition
and in t/db_dependent/Accounts.t
2) Apply the patch
3) Run commands from 1) again, now there should be no occurence
4) prove t/db_dependent/Accounts.t
    should return green

Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>