Removed test_plugin_prescript.py
Removed test_plugin_prescript.py

Packstack gerrit gating does not check unit tests on RHEL.
Because of that the only test in `test_plugin_prescript.py`
is not failing. The test name is `test_rhn_creds_quoted`.
On non-RHEL systems test is simply skipped.

Currently the test does not make any sense to exist, hence
the removal.

Fixes-Bug: #1694469
Change-Id: I0e65cd76f5005edf0074cd378b2314298a0670db
Signed-off-by: Maciej Kucia <33c1fdf481c8e628d4c6db7ea8dc77f49f2fa5d7@kucia.net>

fr34k8/packstack,stackforge/packstack,openstack/packstack,openstack/packstack,fr34k8/packstack,stackforge/packstack,fr34k8/packstack,stackforge/packstack,mahak/packstack,mahak/packstack,mahak/packstack,openstack/packstack