Remove DoubleCheckedLocking check as it has been removed since checkstyle 5.6

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/email/trunk@1532470 13f79535-47bb-0310-9956-ffa450edef68
Prepare for RC1.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/email/trunk@1532471 13f79535-47bb-0310-9956-ffa450edef68