freertos-integration-tests: Remove out-of-tree patch

* Remove upstreamed out of tree patch
  `0003-Introduce-user-defined-MQTT-keep-alive-value.patch`.
* With the latest version of FreeRTOS Integration tests, OTA PAL test
  has been removed. Therefore, remove OTA PAL test specific changes from
  integration layer including the patch
  0002-closefile-validsignature-test-Fix-test-bugs.patch.

Signed-off-by: Devaraj Ranganna <devaraj.ranganna@arm.com>