Bug 20510: Remove unused sub TotalPaid from C4::Stats

Bug 20510: Remove unused sub TotalPaid from C4::Stats

Test plan:
0) Do not apply the patch
1) Run git grep TotalPaid and confirm it is only defined and then used once in a test
2) Apply the patch
3) git grep TotalPaid should return nothing
4) prove t/db_dependent/Stats.t should be green

Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>