Revert "Don't allow XContentBuilder#writeValue(TimeValue)"

This reverts commit 7f70c00dad.

core/src/main/java/org/elasticsearch/common/xcontent/XContentBuilder.java

@@ -886,7 +886,6 @@ public final class XContentBuilder implements BytesStream, Releasable {
             generator.writeNull();
             return;
         }
-        assert false == value instanceof TimeValue : "Use timeValueField instead";
         Class<?> type = value.getClass();
         Writer writer = MAP.get(type);
         if (writer != null) {