Fix unit tests on Java 11 (#1483)

The unit test I18nServlet and MultiParentClassLoaderTest are failing on Java 11.
This is because they mock ClassLoaders which cause a jvm error.

The following tickets describe the problem in more detail:

- https://bugs.openjdk.java.net/browse/JDK-8254969
- https://github.com/mockito/mockito/issues/2043
- https://github.com/mockito/mockito/issues/1696

scm-webapp/src/main/java/sonia/scm/web/i18n/I18nServlet.java

@@ -138,7 +138,6 @@ public class I18nServlet extends HttpServlet {
     return context.getStage() == Stage.PRODUCTION;
   }
 
-  @VisibleForTesting
   private Optional<JsonNode> collectJsonFile(String path) throws IOException {
     LOG.debug("Collect plugin translations from path {} for every plugin", path);
     JsonNode mergedJsonNode = null;