Bug 23215: Remove traces of the PayPal feature

Bug 23215: Remove traces of the PayPal feature

This patch removes the PayPal payments feature. It has been moved to
its' own plugin.

To test:
1. Apply this patches
2. Run:
   $ updatedatabase
=> SUCCESS: Database updates correctly
3. Run:
   $ koha-mysql kohadev
   > SELECT * FROM systempreferences WHERE variable LIKE 'paypal';
=> SUCCESS: No results
4. On the sysprefs, OPAC section
=> SUCCESS: No PayPal-related sysprefs show up
5. Add some charges to your patron
6. In the OPAC, log in and see your charges
=> SUCCESS: Nothing broken
7. Install the PayPal plugin [1] or any other payment plugin
8. Restart plack (mandatory for the PayPal plugin)
9. Set some random data in the config (or better, real sandbox testing
   data)
10. Go to the OPAC's account page and try to pay your debts (use the
    checkbox to select lines)
=> SUCCESS: The PayPal payment method shows, you can click the button,
    it fails due to bad config, but things work as expected.
11. Sign off :-D

[1] https://gitlab.com/thekesolutions/plugins/koha-plugin-pay-via-paypal/-/releases

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>