Bug 28374: Convert printreceipt to use GetPreparedLetter

Bug 28374: Convert printreceipt to use GetPreparedLetter

This patch converts the point of sale receipt printer controller to
using GetPreparedLetter instead of calling getletter directly.

Test plan
1. Setup for testing:
   i.   Enable EnablePointOfSale and UseCashRegisters system preferences
   ii.  Add a new cash register
        (Administration > Accounting > Cash registers)
   iii. Add one or two products for sale
        (Administration > Accounting > Debit types, make sure 'Can be sold?' is ticked)
2. Go to Home > Point of sale
3. Add some items for purchase to a sale
4. Click confirm
5. Click on 'Print receipt' and save the PDF file somewhere
6. Apply patches
7. Update database: updatedatabase
8. Repeat steps 2-5
9. Compare receipt PDFs - should be no changes
10. Check that the default RECEIPT is updated - for print, HTML should be ticked
11. Sign off!

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>