Review service "Customer" in the OnPremise Jira client

In #473 we split the single Jira client into two: OnPremise and Cloud.
We did this by copying the same code into two folders. The reason: The two software products have different APIs.
In v1 of this client, we had a bad/dirty mix of OnPremise and Cloud API endpoints.

## Goal of this task

Review service "Customer" in the OnPremise Jira client and

1. delete all API endpoints related to the Cloud Jira version
2. update the documentation to match the latest API links

## Notes

This might reduce the functionality of the client, but this lead to a more stable client.

Missing API endpoints will be documented that we have a clear overview.