Improve a bit the changelog.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1586955 13f79535-47bb-0310-9956-ffa450edef68
Followup to r1586897 :
Apply the same not-null optimization to removeAttributeChangeNotificationListener method.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1586959 13f79535-47bb-0310-9956-ffa450edef68