Improve Elmax code quality

[albertogeniola]

Proposed change

The Elmax integration has been merged in #59321, but some improvements/edited have been suggested by reviewers. This PR addresses such requests. In particular this PR changes the following:

• Removes useless index & visibility extra attributes from ElmaxEntity
• Simplifies the ElmaxEntity logic by leveraging the CoordinatorEntity (Thanks @frenck !)
• Simplifies tests logic (thanks @MartinHjelmare !)

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 is related to previous PR: Elmax integration #59321

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.

[MartinHjelmare]

Looks good!

[albertogeniola]

Looks good!

Thanks Martin for your review!

[albertogeniola]

I just added a few more commits that have been pending from a previous review. We should finally be on the right track this time!

[frenck]

Nice! Thanks, @albertogeniola 👍

[albertogeniola]

Thanks to you guys for the great help you provided.