`FormTest`: Remove mocke `Icinga\Web\Session` in test `testWhetherACsrfCounterMeasureIsBeingAdded()`

Since a fake session is already being instantiated in `BaseTestCase` there is no need to mock the class `Icinga\Web\Session`.

test/php/library/Icinga/Web/FormTest.php

@@ -120,8 +120,6 @@ class FormTest extends BaseTestCase
      */
     public function testWhetherACsrfCounterMeasureIsBeingAdded()
     {
-        Mockery::mock('alias:Icinga\Web\Session')->shouldReceive('getSession->getId')->andReturn('1234567890');
-
         $form = new Form();
         $form->create();