Merged r1586959 from tomcat/trunk:
Followup to r1586897 :
Apply the same not-null optimization to removeAttributeChangeNotificationListener method.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/tc7.0.x/trunk@1586960 13f79535-47bb-0310-9956-ffa450edef68