Remove broken functional test folder

Rationale:

1. `tox.ini` doesn't have a `func` target so probably these tests haven't
   been run for a long time.

2. These tests can't be run at the moment because of conflicting version
   ranges in python deps pulled via `requirements.txt` and `setup.py`, e.g.

   error: PyYAML 3.13 is installed but pyyaml<=6.0,>=5.1.2 is required by
          {'juju'}
   error: python-keystoneclient 3.21.0 is installed but
          python-keystoneclient>=3.22.0 is required by
          {'python-openstackclient'}

3. These tests don't have expectations. They just deploy a bundle.

tests/README.md

@@ -1,2 +0,0 @@
-This directory contains functional test definition for functional test of Zaza
-itself.

tests/bundles/magpie.yaml

@@ -1,5 +0,0 @@
-series: bionic
-applications:
-  magpie:
-    charm: cs:~admcleod/magpie
-    num_units: 2

tests/bundles/overlays/local-charm-overlay.yaml.j2

@@ -1 +0,0 @@
-comment: this bundle overlay intentionally left blank

tests/tests.yaml

@@ -1,11 +0,0 @@
-charm_name: none
-gate_bundles:
-- magpie
-target_deploy_status:
-  magpie:
-    workload-status: active
-    workload-status-message: icmp ok, local hostname ok, dns ok
-configure:
-- zaza.openstack.charm_tests.noop.setup.basic_setup
-tests:
-- zaza.openstack.charm_tests.noop.tests.NoopTest