PayPal: Migrate to Order v2 API and ISU authentication (#2493)

Co-authored-by: Raphael Michel <michel@rami.io>

src/pretix/plugins/paypal/apps.py

@@ -33,7 +33,6 @@
 # License for the specific language governing permissions and limitations under the License.
 
 from django.apps import AppConfig
-from django.utils.functional import cached_property
 from django.utils.translation import gettext_lazy as _
 
 from pretix import __version__ as version
@@ -55,12 +54,3 @@ class PaypalApp(AppConfig):
 
     def ready(self):
         from . import signals  # NOQA
-
-    @cached_property
-    def compatibility_errors(self):
-        errs = []
-        try:
-            import paypalrestsdk  # NOQA
-        except ImportError:
-            errs.append("Python package 'paypalrestsdk' is not installed.")
-        return errs