Remove DoubleCheckedLocking check from Checkstyle config

The DoubleCheckedLocking check is not supported by Checkstyle 5.6 which is the
default version used in Gradle 1.4.

Also see http://checkstyle.sourceforge.net/releasenotes.html#Release_5.6 for the
release notes of Checkstyle 5.6.

Author: joschi

codequality/checkstyle.xml

@@ -128,7 +128,6 @@
         <!-- Checks for common coding problems               -->
         <!-- See http://checkstyle.sf.net/config_coding.html -->
         <!-- <module name="AvoidInlineConditionals"/> -->
-        <module name="DoubleCheckedLocking"/>    <!-- MY FAVOURITE -->
         <module name="EmptyStatement"/>
         <module name="EqualsHashCode"/>
         <module name="HiddenField">