WrappedFilterConfig: Fix 'unchecked or unsafe operations' warning

- Use Collections.emptyEnumeration() which is available in Java 7,
  rather than the Collections.enumeration(Collections.emptyList())
  workaround that was used for Java 6 compatibility.

- Add type specification to prevent "unchecked conversion" warning.

- Remove @Suppress annotations that are no longer necessary.

Change-Id: Iba4b61330d5ca8a822b8121fe46fb20418e79347

gerrit-httpd/src/main/java/com/google/gerrit/httpd/plugins/WrappedFilterConfig.java

@@ -39,10 +39,9 @@ class WrappedFilterConfig implements FilterConfig {
     return null;
   }
 
-  @SuppressWarnings({"rawtypes", "unchecked"})
   @Override
-  public Enumeration getInitParameterNames() {
-    return Collections.enumeration(Collections.emptyList());
+  public Enumeration<String> getInitParameterNames() {
+    return Collections.emptyEnumeration();
   }
 
   @Override