refactor(ngView): remove previousElement bookkeeping

it's unnecessary and inconsistent (because finishing animations reset  we see situations
where not previousElement is not always removed and more than one leave animation is going on at the same time).

Closes angular#7606
Closes angular#9355

Author: IgorMinar

src/ngRoute/directive/ngView.js

@@ -188,27 +188,19 @@ function ngViewFactory(   $route,   $anchorScroll,   $animate) {
     link: function(scope, $element, attr, ctrl, $transclude) {
         var currentScope,
             currentElement,
-            previousElement,
             autoScrollExp = attr.autoscroll,
             onloadExp = attr.onload || '';
 
         scope.$on('$routeChangeSuccess', update);
         update();
 
         function cleanupLastView() {
-          if(previousElement) {
-            previousElement.remove();
-            previousElement = null;
-          }
           if(currentScope) {
             currentScope.$destroy();
             currentScope = null;
           }
           if(currentElement) {
-            $animate.leave(currentElement).then(function() {
-              previousElement = null;
-            });
-            previousElement = currentElement;
+            $animate.leave(currentElement);
             currentElement = null;
           }
         }

test/ngRoute/directive/ngViewSpec.js

@@ -844,48 +844,6 @@ describe('ngView animations', function() {
         }
       });
     });
-
-    it('should destroy the previous leave animation if a new one takes place', function() {
-      module(function($provide) {
-        $provide.decorator('$animate', function($delegate, $$q) {
-          var emptyPromise = $$q.defer().promise;
-          $delegate.leave = function() {
-            return emptyPromise;
-          };
-          return $delegate;
-        });
-      });
-      inject(function ($compile, $rootScope, $animate, $location) {
-        var item;
-        var $scope = $rootScope.$new();
-        element = $compile(html(
-          '<div>' +
-            '<div ng-view></div>' +
-          '</div>'
-        ))($scope);
-
-        $scope.$apply('value = true');
-
-        $location.path('/bar');
-        $rootScope.$digest();
-
-        var destroyed, inner = element.children(0);
-        inner.on('$destroy', function() {
-          destroyed = true;
-        });
-
-        $location.path('/foo');
-        $rootScope.$digest();
-
-        $location.path('/bar');
-        $rootScope.$digest();
-
-        $location.path('/bar');
-        $rootScope.$digest();
-
-        expect(destroyed).toBe(true);
-      });
-    });
   });