Misc ApplicationError fixes

* Remove the synchronized keyword from the two methods that are
  using the Lombok Synchronized annotation, since that defeats
  the purpose.
* Remove VisibleForTesting from clearPersistentHostInformation
  because it is used by the ApplicationErrorExtension, which
  here is technically production code.

src/main/java/org/kiwiproject/dropwizard/error/model/ApplicationError.java

@@ -10,7 +10,6 @@
 import static org.kiwiproject.base.KiwiThrowables.nextCauseOfNullable;
 import static org.kiwiproject.base.KiwiThrowables.throwableInfoOfNullable;
 
-import com.google.common.annotations.VisibleForTesting;
 import lombok.AccessLevel;
 import lombok.AllArgsConstructor;
 import lombok.Builder;
@@ -132,7 +131,7 @@ public static void setPersistentHostInformation(String hostName, String ipAddres
      * {@link #checkPersistentHostState()} for the error message to be correct.
      */
     @Synchronized
-    public static synchronized void setPersistentHostInformation(PersistentHostInformation hostInfo) {
+    public static void setPersistentHostInformation(PersistentHostInformation hostInfo) {
         checkArgumentNotNull(hostInfo);
         setPersistentHostInformation(hostInfo.getHostName(), hostInfo.getIpAddress(), hostInfo.getPort());
     }
@@ -146,8 +145,7 @@ public static synchronized void setPersistentHostInformation(PersistentHostInfor
      * persistent host information at the same time.
      */
     @Synchronized
-    @VisibleForTesting
-    public static synchronized void clearPersistentHostInformation() {
+    public static void clearPersistentHostInformation() {
         persistentHostInformation = null;
     }