Remove unneeded tests for StoreKit2 with JWS (#3747)

### Motivation

Unlock shipping by skipping tests that are flaky unneeded tests

### Description

Removing StoreKit2JWS tests from
`BackendIntegrationTests-SK2.xctestplan`

⚠️ The functionality in `StoreKit2JWSIntegrationTests` (which is the
`usesStoreKit2JWS` dangerous setting), is not publicly available in iOS
v4. Its intention was intended only for iOS v5. These tests are safe to
skip since they have no production use.

⚠️ It _is_ important that these are not skipped for iOS v5.

BackendIntegrationTests/BackendIntegrationTests-SK2.xctestplan

@@ -59,6 +59,9 @@
         "StoreKit1IntegrationTests",
         "StoreKit1ObserverModeIntegrationTests",
         "StoreKit1ObserverModeWithExistingPurchasesTests",
+        "StoreKit2JWSIntegrationTests",
+        "StoreKit2JWSObserverModeIntegrationTests",
+        "StoreKit2JWSObserverModeWithExistingPurchasesTests",
         "SubscriberAttributesManagerIntegrationTests",
         "TestCase"
       ],