Remove the unused 'verbose' config option

This config option is provided in one of the base layers, but isn't used
in the charm, resulting in an unexpected non-error when trying to use
it.

Charm-tools' build function has a feature in the layer.yaml to remove a
config key, and this patchset uses that to remove the verbose config
option.

Change-Id: I0daadc63bc37037ae0a60c0c40f39846b7d1783a
Closes-Bug: 1654671

src/layer.yaml

@@ -4,3 +4,6 @@ options:
     use_venv: True
     include_system_packages: True
 repo: https://github.com/openstack/charm-aodh
+config:
+  deletes:
+    - verbose