Nello removal

[pattyland]

Breaking change

Removes Nello integration.

Proposed change

After Locumi Labs GmbH, the manufacturer of Nello, went bankrupt in 2019, SCLAK continued to operate the servers until June 2021. According to unofficial sources, SCLAK also went bankrupt at the beginning of 2021 and was sold.
For half a year now, the MQTT server with which the door opener connects is not accessible, the homepage (https://www.nello.io/) is down and also the API (https://public-api.nello.io/v1/) has long delivered only 500 errors.
I think it's time to dispose of the integration and the hardware. :(

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New integration (thank you!)
• New feature (which adds functionality to an existing integration)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to issue:
• Link to documentation pull request: Nello Removal home-assistant.io#19823

Checklist

• The code change is tested and works locally.
• Local tests pass. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.
• I have followed the development checklist
• The code has been formatted using Black (black --fast homeassistant tests)
• Tests have been added to verify that the new code works.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated for www.home-assistant.io

If the code communicates with devices, web services, or third-party tools:

• The manifest file has all fields filled out correctly.
  Updated and included derived files by running: python3 -m script.hassfest.
• New or updated dependencies have been added to requirements_all.txt.
  Updated by running python3 -m script.gen_requirements_all.
• For the updated dependencies - a link to the changelog, or at minimum a diff between library versions is added to the PR description.
• Untested files have been added to .coveragerc.

The integration reached or maintains the following Integration Quality Scale:

• No score or internal
• 🥈 Silver
• 🥇 Gold
• 🏆 Platinum

To help with the load of incoming pull requests:

• I have reviewed two other open pull requests in this repository.

[probot-home-assistant]

Hey there @pschmitt, mind taking a look at this pull request as it has been labeled with an integration (nello) you are listed as a code owner for? Thanks!
_{^{(message by CodeOwnersMention)}}

[frenck]

Rebased the PR to deal with the CI issues (hopefully).

[frenck]

Thanks, @pattyland 👍