Remove model count based on cache.

[anastasiamac]

Description of change

We were reporting redundant and incorrect information to the user when they were trying to access information about a 'currently in-focus' model that no longer exists. Since we are directing users to 'juju models', which will correctly display all accessible models on the current controller, there is no longer a need to tell users how many models were previously accessed to on this controller. This model count get quickly out of date, especially when models are being created/destroyed using different clients.

This PR removes misleading part of the error message.

QA steps

1. [Client Machine A] bootstrap
2. [Client Machine A] add model 'trial'
3. [Client Machine A] add model 'trial2' [this model is now 'in-focus' on this client machine, considered 'current']
4. [Client Machine B] destroy model 'trial2'
5. [Client Machine A] run 'juju status'

Expected error message:
WARNING cannot read current model: current model for controller not found
ERROR model <full name for 'trial2' model> has been removed from the controller, run 'juju models' and switch to one of them.

Documentation changes

If this error message has been mentioned in documents explicitly, it will need to be updated.

Bug reference

https://bugs.launchpad.net/juju/+bug/1693843

[anastasiamac]

$$merge$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju