[QA] Clean-up testing app unused code

[amrita-shrestha]

In the past, testing app has been used to provide helper function in a centralized location for owncloud 10 , ocis, desktop client.
Nowadays we have helpers in its own repo so testing app has been no used. So, removing code related to testing app will not effect ocis test code base.

In this test folder you can found hint about what helper function/feature step is used by ocis.
https://github.com/owncloud/testing/tree/master/tests/acceptance/features