Remove deprecated Mastodon yaml config import

[andrew-codechimp]

Breaking change

The Mastodon integration moved to config flow in 2024.8 with a deprecation of YAML config scheduled for removal in 2025.2
This PR removes the YAML config import

Proposed change

Tidying up the Mastodon integration by removing now redundant code.

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)
• Deprecation (breaking change to happen in the future)
• 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:

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[home-assistant]

Hey there @fabaff, mind taking a look at this pull request as it has been labeled with an integration (mastodon) you are listed as a code owner for? Thanks!

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[gjohansson-ST]

Why do we remove this? I don't know when it was implemented but it should be in a separate PR in that case.

[andrew-codechimp]

I shouldn't have removed that, got carried away with anything that said migrate.
I've added this back.

[home-assistant]

Please take a look at the requested changes, and use the Ready for review button when you are done, thanks 👍

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[gjohansson-ST]

Thanks @andrew-codechimp 👍