Remove DEC_2015 & APRIL_2015 from the CompatibleVersionConstants

lib/common/util/constants.js

@@ -2149,21 +2149,6 @@ var Constants = {
   },
 
   CompatibleVersionConstants: {
-    /**
-    * Constant for the 2015-12-11 version.
-    *
-    * @const
-    * @type {string}
-    */
-    DEC_2015: '2015-12-11',
-    /**
-    * Constant for the 2015-04-05 version.
-    *
-    * @const
-    * @type {string}
-    */
-    APRIL_2015: '2015-04-05',
-    
     /**
     * Constant for the 2013-08-15 version.
     *