Bug 22394: Remove C4::Accounts::manualinvoice

Bug 22394: Remove C4::Accounts::manualinvoice

This patch removes C4::Accounts::manualinvoice and it's corresponding
test.  We have been warning deprecation of this method for 18 months (by
the time of 20.11 release).

Test plan
1/ Apply patch
2/ Ensure no mention of manualinvoice accross the codebase
3/ Run t/db_dependant/Accounts.t and ensure it still passes

Signed-off-by: Joonas Kylmälä <joonas.kylmala@helsinki.fi>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>