chore(datepicker): Unregister parent watchers on $destroy

Closes angular-ui#5242

src/datepicker/datepicker.js

@@ -251,7 +251,10 @@ angular.module('ui.bootstrap.datepicker', ['ui.bootstrap.dateparser', 'ui.bootst
   };
 
   $scope.$on("$destroy", function() {
-    clearWatchListeners(watchListeners);
+    //Clear all watch listeners on destroy
+    while (watchListeners.length) {
+      watchListeners.shift()();
+    }
   });
 }])
 
@@ -776,7 +779,9 @@ function(scope, element, attrs, $compile, $parse, $document, $rootScope, $positi
       $document.unbind('click', documentClickBind);
 
       //Clear all watch listeners on destroy
-      clearWatchListeners(watchListeners);
+      while (watchListeners.length) {
+        watchListeners.shift()();
+      }
     });
   };
 
@@ -997,9 +1002,3 @@ function(scope, element, attrs, $compile, $parse, $document, $rootScope, $positi
     }
   };
 });
-
-function clearWatchListeners(watchListeners) {
-  for (var i = 0; i < watchListeners.length; i++) {
-    watchListeners[i]();
-  }
-}