Remove ansible<2.2 apt cache hack

Now ansible apt module correctly behaves, so it's time
to deprecate these cruft tasks for apt.

Change-Id: I8c6fe61eca963aae5bbda5cf81481c420b952a37

tasks/rhel6stig/apt.yml

@@ -13,20 +13,10 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-#TODO(evrardjp): Replace the next 2 tasks by a standard apt with cache
-#when https://github.com/ansible/ansible-modules-core/pull/1517 is merged
-#in 1.9.x or we move to 2.0 (if tested working)
-- name: Check apt last update file
-  stat:
-    path: /var/cache/apt
-  register: apt_cache_stat
-  tags:
-    - auditd-apt-packages
-
 - name: Update apt if needed
   apt:
     update_cache: yes
-  when: "ansible_date_time.epoch|float - apt_cache_stat.stat.mtime > {{cache_timeout}}"
+    cache_valid_time: "{{ cache_timeout }}"
   tags:
     - auditd-apt-packages